Implement NetworkService request interception/handling (see issue #2622).

Implementation notes:
- Chromium change: CookieMonster::SetCookieableSchemes needs to be called
  immediately after the CookieMonster is created in NetworkContext::
  ApplyContextParamsToBuilder. Add a Profile::GetCookieableSchemes method and
  NetworkContextParams.cookieable_schemes member (set from
  ProfileNetworkContextService::CreateNetworkContextParams) to support that.
- Chromium change: Add a ContentBrowserClient::HandleExternalProtocol variant
  that exposes additional NetworkService request information.
- GetResourceResponseFilter is not yet implemented.

API changes:
- Resource-related callbacks have been moved from CefRequestHandler to a new
  CefResourceRequestHandler interface which is returned via the
  GetResourceRequestHandler method. If the CefRequestHandler declines to handle
  a resource it can optionally be handled by the CefRequestContextHandler, if
  any, associated with the loading context.
- The OnProtocolExecution callback has been moved from CefRequestHandler to
  CefResourceRequestHandler and will be called if a custom scheme request is
  unhandled.
- Cookie send/save permission callbacks have been moved from CefRequestHandler
  and CefResourceHandler to CefResourceRequestHandler.
- New methods added to CefResourceHandler that better match NetworkService
  execution sequence expectations. The old methods are now deprecated.
- New methods added to CefRequest and CefResponse.

Known behavior changes with the NetworkService implementation:
- Modifying the |new_url| parameter in OnResourceRedirect will no longer result
  in the method being called an additional time (likely a bug in the old
  implementation).
- Modifying the request URL in OnResourceResponse would previously cause a
  redirect. This behavior is now deprecated because the NetworkService does not
  support this functionality when using default network loaders. Temporary
  support has been added in combination with CefResourceHandler usage only.
- Other changes to the request object in OnResourceResponse will now cause the
  request to be restarted. This means that OnBeforeResourceLoad, etc, will be
  called an additional time with the new request information.
- CefResponse::GetMimeType will now be empty for non-200 responses.
- Requests using custom schemes can now be handled via CefResourceRequestHandler
  with the same callback behavior as builtin schemes.
- Redirects of custom scheme requests will now be followed as expected.
- Default handling of builtin schemes can now be disabled by setting
  |disable_default_handling| to true in GetResourceRequestHandler.
- Unhandled requests (custom scheme or builtin scheme with default handling
  disabled) will fail with an CefResponse::GetError value of
  ERR_UNKNOWN_URL_SCHEME.
- The CefSchemeHandlerFactory::Create callback will now include cookie headers.

To test:
- Run `cefclient --enable-network-service`. All resources should load
  successfully (this tests the transparent proxy capability).
- All tests pass with NetworkService disabled.
- The following tests pass with NetworkService enabled:
  - CookieTest.*
  - FrameTest.* (excluding .*Nav)
  - NavigationTest.* (excluding .Redirect*)
  - RequestHandlerTest.*
  - RequestContextTest.Basic*
  - RequestContextTest.Popup*
  - RequestTest.*
  - ResourceManagerTest.*
  - ResourceRequestHandlerTest.* (excluding .Filter*)
  - SchemeHandlerTest.*
  - StreamResourceHandlerTest.*

include/cef_request.h

@@ -140,6 +140,25 @@ class CefRequest : public virtual CefBaseRefCounted {
   /*--cef()--*/
   virtual void SetHeaderMap(const HeaderMap& headerMap) = 0;
 
+  ///
+  // Returns the first header value for |name| or an empty string if not found.
+  // Will not return the Referer value if any. Use GetHeaderMap instead if
+  // |name| might have multiple values.
+  ///
+  /*--cef()--*/
+  virtual CefString GetHeaderByName(const CefString& name) = 0;
+
+  ///
+  // Set the header |name| to |value|. If |overwrite| is true any existing
+  // values will be replaced with the new value. If |overwrite| is false any
+  // existing values will not be overwritten. The Referer value cannot be set
+  // using this method.
+  ///
+  /*--cef()--*/
+  virtual void SetHeaderByName(const CefString& name,
+                               const CefString& value,
+                               bool overwrite) = 0;
+
   ///
   // Set all values at one time.
   ///
@@ -194,8 +213,8 @@ class CefRequest : public virtual CefBaseRefCounted {
 
   ///
   // Returns the globally unique identifier for this request or 0 if not
-  // specified. Can be used by CefRequestHandler implementations in the browser
-  // process to track a single request across multiple callbacks.
+  // specified. Can be used by CefResourceRequestHandler implementations in the
+  // browser process to track a single request across multiple callbacks.
   ///
   /*--cef()--*/
   virtual uint64 GetIdentifier() = 0;