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.

libcef/browser/browser_main.cc

@@ -210,6 +210,9 @@ void CefBrowserMainParts::PostMainMessageLoopRun() {
   // NOTE: Destroy objects in reverse order of creation.
   CefDevToolsManagerDelegate::StopHttpHandler();
 
+  // There should be no additional references to the global CefRequestContext
+  // during shutdown. Did you forget to release a CefBrowser reference?
+  DCHECK(global_request_context_->HasOneRef());
   global_request_context_ = NULL;
 
   if (extensions::ExtensionsEnabled()) {