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/path_util_impl.cc

@@ -38,6 +38,9 @@ bool CefGetPath(PathKey key, CefString& path) {
     case PK_USER_DATA:
       pref_key = chrome::DIR_USER_DATA;
       break;
+    case PK_DIR_RESOURCES:
+      pref_key = chrome::DIR_RESOURCES;
+      break;
     default:
       NOTREACHED() << "invalid argument";
       return false;