Move message routing from CefBrowser to CefFrame (see issue #2498).

This change moves the SendProcessMessage method from CefBrowser to CefFrame and
adds CefBrowser parameters to OnProcessMessageReceived and
OnDraggableRegionsChanged.

The internal implementation has changed as follows:
- Frame IDs are now a 64-bit combination of the 32-bit render_process_id and
  render_routing_id values that uniquely identify a RenderFrameHost (RFH).
- CefFrameHostImpl objects are now managed by CefBrowserInfo with life span tied
  to RFH expectations. Specifically, a CefFrameHostImpl object representing a
  sub-frame will be created when a RenderFrame is created in the renderer
  process and detached when the associated RenderFrame is deleted or the
  renderer process in which it runs has died.
- The CefFrameHostImpl object representing the main frame will always be valid
  but the underlying RFH (and associated frame ID) may change over time as a
  result of cross-origin navigations. Despite these changes calling LoadURL on
  the main frame object in the browser process will always navigate as expected.
- Speculative RFHs, which may be created as a result of a cross-origin
  navigation and discarded if that navigation is not committed, are now handled
  correctly (e.g. ignored in most cases until they're committed).
- It is less likely, but still possible, to receive a CefFrame object with an
  invalid frame ID (ID < 0). This can happen in cases where a RFH has not yet
  been created for a sub-frame. For example, when OnBeforeBrowse is called
  before initiating navigation in a previously nonexisting sub-frame.

To test: All tests pass with NetworkService enabled and disabled.

libcef/browser/net/net_util.h

@@ -17,12 +17,24 @@ class URLRequest;
 
 class GURL;
 
+class CefBrowserHostImpl;
+class CefBrowserInfo;
+
 namespace net_util {
 
 // Returns true if |request| is handled internally and should not be exposed via
 // the CEF API.
 bool IsInternalRequest(const net::URLRequest* request);
 
+// Returns the browser associated with the specified URLRequest.
+CefRefPtr<CefBrowserHostImpl> GetBrowserForRequest(
+    const net::URLRequest* request);
+
+// Returns the frame associated with the specified URLRequest.
+CefRefPtr<CefFrame> GetFrameForRequest(
+    scoped_refptr<CefBrowserInfo> browser_info,
+    const net::URLRequest* request);
+
 // Returns the appropriate CefResourceRequestHandler as determined by the
 // associated CefBrowser/CefRequestHandler and/or CefRequestContextHandler, if
 // any. The out-params will be nullptr if no handler is returned. Otherwise,