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/views/button_view.h

@@ -56,12 +56,16 @@ CEF_BUTTON_VIEW_T void CEF_BUTTON_VIEW_D::StateChanged(
 CEF_BUTTON_VIEW_T void CEF_BUTTON_VIEW_D::ButtonPressed(
     views::Button* sender,
     const ui::Event& event) {
+  // Callback may trigger new animation state.
   if (ParentClass::cef_delegate())
     ParentClass::cef_delegate()->OnButtonPressed(GetCefButton());
   if (ParentClass::ink_drop_mode() != views::CustomButton::InkDropMode::OFF &&
-      !ParentClass::IsFocusable()) {
-    // When ink drop is enabled for non-focusable buttons the ink drop state
-    // does not get reset properly on click, so we do it here explicitly.
+      !ParentClass::IsFocusable() &&
+      ParentClass::state() != views::CustomButton::STATE_PRESSED) {
+    // Ink drop state does not get reset properly on click when the button is
+    // non-focusable. Reset the ink drop state here if the state has not been
+    // explicitly set to pressed by the OnButtonPressed callback calling
+    // SetState (which also sets the ink drop state).
     ParentClass::AnimateInkDrop(views::InkDropState::HIDDEN,
                                 ui::LocatedEvent::FromIfValid(&event));
   }