Extend the hang monitor timeout when an event ack is received.

The hang monitor is supposed to wait for 30 seconds of inactivity before
deciding that a renderer is not responsive. This CL makes sure that when
waiting for multiple input events, receiving an ack is correctly interpreted
as a responsive renderer (for that moment)

BUG=336198

Review URL: https://codereview.chromium.org/535653002

Cr-Commit-Position: refs/heads/master@{#293060}

content/browser/renderer_host/render_widget_host_impl.cc

@@ -1779,9 +1779,13 @@ void RenderWidgetHostImpl::IncrementInFlightEventCount() {
 
 void RenderWidgetHostImpl::DecrementInFlightEventCount() {
   DCHECK_GE(in_flight_event_count_, 0);
-  // Cancel pending hung renderer checks since the renderer is responsive.
-  if (decrement_in_flight_event_count() <= 0)
+  if (decrement_in_flight_event_count() <= 0) {
+    // Cancel pending hung renderer checks since the renderer is responsive.
     StopHangMonitorTimeout();
+  } else {
+    // The renderer is responsive, but there are in-flight events to wait for.
+    RestartHangMonitorTimeout();
+  }
 }
 
 void RenderWidgetHostImpl::OnHasTouchEventHandlers(bool has_handlers) {