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/cef_request_context_handler.h

@@ -42,6 +42,8 @@
 #include "include/cef_cookie.h"
 #include "include/cef_web_plugin.h"
 
+class CefRequestContext;
+
 ///
 // Implement this interface to provide handler implementations. The handler
 // instance will not be released until all objects related to the context have
@@ -52,6 +54,14 @@ class CefRequestContextHandler : public virtual CefBaseRefCounted {
  public:
   typedef cef_plugin_policy_t PluginPolicy;
 
+  ///
+  // Called on the browser process UI thread immediately after the request
+  // context has been initialized.
+  ///
+  /*--cef()--*/
+  virtual void OnRequestContextInitialized(
+      CefRefPtr<CefRequestContext> request_context) {}
+
   ///
   // Called on the browser process IO thread to retrieve the cookie manager. If
   // this method returns NULL the default cookie manager retrievable via