Don't restart the hang renderer timeout on messages ack'd from the compositor thread.

content/browser/renderer_host/render_widget_host_impl.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/renderer_host/render_widget_host_impl.h"
 
 #include <math.h>
 
 #include <set>
 #include <tuple>
@@ skipping 1888 matching lines @@
 void RenderWidgetHostImpl::IncrementInFlightEventCount(
     blink::WebInputEvent::Type event_type) {
   increment_in_flight_event_count();
   if (!is_hidden_) {
     StartHangMonitorTimeout(
         hung_renderer_delay_, event_type,
         RendererUnresponsiveType::RENDERER_UNRESPONSIVE_IN_FLIGHT_EVENTS);
   }
 }
 
-void RenderWidgetHostImpl::DecrementInFlightEventCount() {
+void RenderWidgetHostImpl::DecrementInFlightEventCount(
+    InputEventAckSource ack_source) {
   if (decrement_in_flight_event_count() <= 0) {
     // Cancel pending hung renderer checks since the renderer is responsive.
     StopHangMonitorTimeout();
   } else {
-    RestartHangMonitorTimeoutIfNecessary();
+    // Only restart the hang monitor timer if we got a response from the
+    // main thread.
+    if (ack_source == InputEventAckSource::MAIN_THREAD)
+      RestartHangMonitorTimeoutIfNecessary();
   }
 }
 
 void RenderWidgetHostImpl::OnHasTouchEventHandlers(bool has_handlers) {
   has_touch_handler_ = has_handlers;
 }
 
 void RenderWidgetHostImpl::DidFlush() {
   if (synthetic_gesture_controller_)
     synthetic_gesture_controller_->OnDidFlushInput();
@@ skipping 290 matching lines @@
   return delegate_ ? delegate_->GetRootBrowserAccessibilityManager() : NULL;
 }
 
 BrowserAccessibilityManager*
     RenderWidgetHostImpl::GetOrCreateRootBrowserAccessibilityManager() {
   return delegate_ ?
       delegate_->GetOrCreateRootBrowserAccessibilityManager() : NULL;
 }
 
 }  // namespace content