Add support for loading extensions (issue #1947)

- Add CefRequestContext::LoadExtension, CefExtension, CefExtensionHandler and
  related methods/interfaces.
- Add chrome://extensions-support that lists supported Chrome APIs.
- Add CefBrowserHost::SetAutoResizeEnabled and CefDisplayHandler::OnAutoResize
  to support browser resize based on preferred web contents size.
- views: Add support for custom CefMenuButton popups.
- cefclient: Run with `--load-extension=set_page_color` command-line flag for
  an extension loading example. Add `--use-views` on Windows and Linux for an
  even better example.

include/capi/cef_request_context_handler_capi.h

@@ -33,7 +33,7 @@
 // by hand. See the translator.README.txt file in the tools directory for
 // more information.
 //
-// $hash=9359e227c9d534c9c612d2ede790136461836501$
+// $hash=5a72321dd65325d93c1f920b08fc6bd462e74bdf$
 //
 
 #ifndef CEF_INCLUDE_CAPI_CEF_REQUEST_CONTEXT_HANDLER_CAPI_H_
@@ -48,6 +48,8 @@
 extern "C" {
 #endif
 
+struct _cef_request_context_t;
+
 ///
 // Implement this structure to provide handler implementations. The handler
 // instance will not be released until all objects related to the context have
@@ -59,6 +61,14 @@ typedef struct _cef_request_context_handler_t {
   ///
   cef_base_ref_counted_t base;
 
+  ///
+  // Called on the browser process UI thread immediately after the request
+  // context has been initialized.
+  ///
+  void(CEF_CALLBACK* on_request_context_initialized)(
+      struct _cef_request_context_handler_t* self,
+      struct _cef_request_context_t* request_context);
+
   ///
   // Called on the browser process IO thread to retrieve the cookie manager. If
   // this function returns NULL the default cookie manager retrievable via