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/os_rendering_unittest.cc

@@ -969,14 +969,14 @@ class OSRTestHandler : public RoutingTestHandler,
     return false;
   }
 
-  void OnTakeFocus(CefRefPtr<CefBrowser> browser, bool next) {
+  void OnTakeFocus(CefRefPtr<CefBrowser> browser, bool next) override {
     if (test_type_ == OSR_TEST_TAKE_FOCUS) {
       EXPECT_TRUE(true);
       DestroySucceededTestSoon();
     }
   }
 
-  void OnGotFocus(CefRefPtr<CefBrowser> browser) {
+  void OnGotFocus(CefRefPtr<CefBrowser> browser) override {
     if (test_type_ == OSR_TEST_GOT_FOCUS) {
       EXPECT_TRUE(true);
       DestroySucceededTestSoon();