Make event-targeting asynchronous in window server.

services/ui/ws/window_manager_state.cc

Index: services/ui/ws/window_manager_state.cc
diff --git a/services/ui/ws/window_manager_state.cc b/services/ui/ws/window_manager_state.cc
index 00347db7ad2cc15ee378c6f95c68931436a10dd7..4ab74286c575d52fad76b74f89dff822ca978b6f 100644
--- a/services/ui/ws/window_manager_state.cc
+++ b/services/ui/ws/window_manager_state.cc
@@ -290,7 +290,7 @@ void WindowManagerState::ProcessEvent(const ui::Event& event,
                                       int64_t display_id) {
   // If this is still waiting for an ack from a previously sent event, then
   // queue up the event to be dispatched once the ack is received.
-  if (in_flight_event_details_) {
+  if (in_flight_event_details_ || event_dispatcher_.HittestInFlight()) {

[sky, 2017/05/15 21:20:23]

The code you have here will only work if EventDispatchDelegate calls to its
delegate when the queue is empty so that WMS knows to start processing the next
event.

[riajiang, 2017/05/17 02:01:59]

Actually I was looking at |in_flight_event_details_|
(https://cs.chromium.org/chromium/src/services/ui/ws/window_manager_state.h?ty...)
again and was wondering if it's true that it would be non-null until after we've
successfully dispatched the event to the client? (cuz it seems like we only call
ProcessNextEventFromQueue() when we are done processing+dispatching the event)
If that is true, then it means if there's a hit-test in flight in
EventDispatcher, the |in_flight_event_details_| would be non-null so it would
already go inside this if-block without checking IsHitTestInFlight()? 

[sky, 2017/05/17 18:14:43]

in_flight_event_details_ is only created from callbacks from EventDispatcher
(DispatchInputEventToWindow for one). So, if calling to EventDispatcher doesn't
immediately call to DispatchInputEventToWindow() in_flight_event_details_ won't
be set to a non-null value.

[riajiang, 2017/05/19 16:49:30]

I ended up keeping event_dispatcher_.IsHitTestInFlight() and notifying
WindowManagerState when hit-tests are done in EventDispatcher. Now the queue for
event targeting and event dispatching is in WindowManagerState and the queue for
other hit-test queries is in EventDispatcher. PTAL, thanks!

     if (!event_queue_.empty() && !event_queue_.back()->processed_target &&
         EventsCanBeCoalesced(*event_queue_.back()->event, event)) {
       event_queue_.back()->event = CoalesceEvents(