Add telemetry to track the time an event spent waiting for the main thread.

content/common/browser_rendering_stats.h

Index: content/common/browser_rendering_stats.h
diff --git a/content/common/browser_rendering_stats.h b/content/common/browser_rendering_stats.h
index 1db8dd05d03d061a3c33a357ff03ac4592082297..72b19fc55d2f92736c4882f71a7ce61508846953 100644
--- a/content/common/browser_rendering_stats.h
+++ b/content/common/browser_rendering_stats.h
@@ -25,6 +25,11 @@ struct CONTENT_EXPORT BrowserRenderingStats {
 
   uint32 scroll_update_count;
   base::TimeDelta total_scroll_update_latency;
+
+  uint32 impl_to_main_touch_event_count;
+  base::TimeDelta total_impl_to_main_touch_event_latency;
+  uint32 impl_to_main_gesture_scroll_event_count;
+  base::TimeDelta total_impl_to_main_gesture_scroll_event_latency;
   // Note: when adding new members, please remember to update enumerateFields
   // in browser_rendering_stats.cc.