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.

tests/ceftests/webui_unittest.cc

@@ -166,6 +166,7 @@ WEBUI_TEST(appcache_internals);
 WEBUI_TEST(accessibility);
 WEBUI_TEST(blob_internals);
 WEBUI_TEST(credits);
+WEBUI_TEST(extensions_support);
 WEBUI_TEST(gpu);
 WEBUI_TEST(histograms);
 WEBUI_TEST(indexeddb_internals);