Coalesce async touch move events until the ack back from render

content/browser/renderer_host/input/touch_event_queue.h

Index: content/browser/renderer_host/input/touch_event_queue.h
diff --git a/content/browser/renderer_host/input/touch_event_queue.h b/content/browser/renderer_host/input/touch_event_queue.h
index a61b90d8eca8c1707d31b3e9bfbadabb175047a0..ef91177c0eef2d2c906e71dfd59e715467dca333 100644
--- a/content/browser/renderer_host/input/touch_event_queue.h
+++ b/content/browser/renderer_host/input/touch_event_queue.h
@@ -66,6 +66,12 @@ class CONTENT_EXPORT TouchEventQueue {
   void ProcessTouchAck(InputEventAckState ack_result,
                        const ui::LatencyInfo& latency_info);
 
+  // Notifies the queue that an uncancelable touchmove event has been received
+  // by the renderer. The queue will decrease the uncancelable pending
+  // touchmove ack count and decide if it should dispatch the next pending
+  // async touch move event.
+  void ProcessUncancelableTouchMoveAck();
+
   // When GestureScrollBegin is received, we send a touch cancel to renderer,
   // route all the following touch events directly to client, and ignore the
   // ack for the touch cancel. When Gesture{ScrollEnd,FlingStart} is received,
@@ -102,6 +108,10 @@ class CONTENT_EXPORT TouchEventQueue {
 
   bool has_handlers() const { return has_handlers_; }
 
+  size_t uncancelable_touch_moves_pending_ack_count() const {
+    return ack_pending_async_touchmove_.size();
+  }
+
  private:
   class TouchTimeoutHandler;
   class TouchMoveSlopSuppressor;
@@ -202,6 +212,16 @@ class CONTENT_EXPORT TouchEventQueue {
   // For details see the design doc at http://goo.gl/lVyJAa.
   bool send_touch_events_async_;
   scoped_ptr<TouchEventWithLatencyInfo> pending_async_touchmove_;
+
+  // For uncancelable touch moves, not only we send a fake ack, but also a real
+  // ack from render, which we use to decide when to send the next async
+  // touchmove. This can help avoid the touch event queue keep growing when
+  // render handles touchmove slow. We use a queue ack_pending_async_touchmove_
+  // to store the recent dispatched uncancelable touchmoves which are still
+  // waiting for their acks back from render. We do not put them back to the
+  // front the touch_event_queue any more.
+  std::deque<TouchEventWithLatencyInfo> ack_pending_async_touchmove_;
+
   double last_sent_touch_timestamp_sec_;
 
   // Event is saved to compare pointer positions for new touchmove events.