Don't use PostTask queueing between compositor and main thread.

content/renderer/input/main_thread_event_queue.h

Index: content/renderer/input/main_thread_event_queue.h
diff --git a/content/renderer/input/main_thread_event_queue.h b/content/renderer/input/main_thread_event_queue.h
index 1417771c141d33893bfbcd89b39305136263fb9b..62350cfa0c68ad243954024abaf05a95259a1968 100644
--- a/content/renderer/input/main_thread_event_queue.h
+++ b/content/renderer/input/main_thread_event_queue.h
@@ -42,13 +42,14 @@ class EventWithDispatchType : public ScopedWebInputEventWithLatencyInfo {
 
 class CONTENT_EXPORT MainThreadEventQueueClient {
  public:
-  // Send an |event| that was previously queued (possibly
+  // Handle an |event| that was previously queued (possibly
   // coalesced with another event) to the |routing_id|'s
   // channel. Implementors must implement this callback.
-  virtual void SendEventToMainThread(int routing_id,
-                                     const blink::WebInputEvent* event,
-                                     const ui::LatencyInfo& latency,
-                                     InputEventDispatchType dispatch_type) = 0;
+  virtual void HandleEventOnMainThread(
+      int routing_id,
+      const blink::WebInputEvent* event,
+      const ui::LatencyInfo& latency,
+      InputEventDispatchType dispatch_type) = 0;
 
   virtual void SendInputEventAck(int routing_id,
                                  blink::WebInputEvent::Type type,
@@ -92,10 +93,13 @@ class CONTENT_EXPORT MainThreadEventQueueClient {
 //            <--(PT)---     // Notify from BL event.
 //   <-------(ACK)------

[tdresser, 2016/07/27 13:51:51]

This comment needs updating, doesn't it?

[dtapuska, 2016/08/03 20:20:54]

Done.

 //
-class CONTENT_EXPORT MainThreadEventQueue {
+class CONTENT_EXPORT MainThreadEventQueue
+    : public base::RefCountedThreadSafe<MainThreadEventQueue> {
  public:
-  MainThreadEventQueue(int routing_id, MainThreadEventQueueClient* client);
-  ~MainThreadEventQueue();
+  MainThreadEventQueue(
+      int routing_id,
+      MainThreadEventQueueClient* client,
+      const scoped_refptr<base::SingleThreadTaskRunner>& main_task_runner);
 
   // Called once the compositor has handled |event| and indicated that it is
   // a non-blocking event to be queued to the main thread.
@@ -112,6 +116,15 @@ class CONTENT_EXPORT MainThreadEventQueue {
   void set_is_flinging(bool is_flinging) { is_flinging_ = is_flinging; }
 
  private:
+  friend class base::RefCountedThreadSafe<MainThreadEventQueue>;
+  ~MainThreadEventQueue();
+  void QueueEvent(std::unique_ptr<EventWithDispatchType>&& event);
+  void SendEventNotificationToMainThread();
+  void PopEventOnMainThread();
+  void SendEventToMainThread(const blink::WebInputEvent* event,
+                             const ui::LatencyInfo& latency,
+                             InputEventDispatchType original_dispatch_type);
+
   friend class MainThreadEventQueueTest;
   int routing_id_;
   MainThreadEventQueueClient* client_;
@@ -120,6 +133,9 @@ class CONTENT_EXPORT MainThreadEventQueue {
   bool is_flinging_;
   bool sent_notification_to_main_;
 
+  base::Lock event_queue_mutex_;

[tdresser, 2016/07/27 13:51:51]

Looks like event_queue_lock_ would be better in line with other lock naming.

[dtapuska, 2016/08/03 20:20:54]

Done.

+  scoped_refptr<base::SingleThreadTaskRunner> main_task_runner_;
+
   DISALLOW_COPY_AND_ASSIGN(MainThreadEventQueue);
 };