Add plugin placeholder and policy support (issue #1708)

- Default plugin loading policy can be specified using the new
  `--plugin-policy=[allow|block|detect]` command-line flag.
- Move CefRequestHandler::OnBeforePluginLoad to
  CefRequestContextHandler and add a new policy argument that
  supports different actions (allow, block, detect, disable) on a
  per-plugin-instance basis.
- Add CefContextMenuHandler::RunContextMenu for providing a custom
  context menu implementation.
- Add CefResourceBundleHandler::GetDataResourceForScale for
  returning scaled resources (issue #1272).
- Add CefResourceBundle for retrieving resources from the resource
  bundle (*.pak) files loaded by CEF during startup or via the
  CefResourceBundleHandler.
- Linux: Fix Debug build IO access warning with CefGetMimeType.
- cef_unittests: Move the refcounting implementation from TestHandler
  to subclasses in order to support interface inheritance from
  subclasses.

libcef/browser/browser_context_impl.h

@@ -87,6 +87,8 @@ class CefBrowserContextImpl : public CefBrowserContext {
       content::ProtocolHandlerMap* protocol_handlers,
       content::URLRequestInterceptorScopedVector request_interceptors)
       override;
+  HostContentSettingsMap* GetHostContentSettingsMap() override;
+  PrefService* GetPrefs() override;
 
   // Guaranteed to exist once this object has been initialized.
   scoped_refptr<CefURLRequestContextGetterImpl> request_context() const {
@@ -115,6 +117,7 @@ class CefBrowserContextImpl : public CefBrowserContext {
   scoped_refptr<CefURLRequestContextGetterImpl> url_request_getter_;
   scoped_ptr<content::PermissionManager> permission_manager_;
   scoped_ptr<CefSSLHostStateDelegate> ssl_host_state_delegate_;
+  scoped_refptr<HostContentSettingsMap> host_content_settings_map_;
 
   DISALLOW_COPY_AND_ASSIGN(CefBrowserContextImpl);
 };