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

cc/debug/rendering_stats.cc

Index: cc/debug/rendering_stats.cc
diff --git a/cc/debug/rendering_stats.cc b/cc/debug/rendering_stats.cc
index 12c54530c71e573e09e640fd79f5e962f85ed34b..e3a9abb2ff55dce1c71d573a450c9b774ef68e46 100644
--- a/cc/debug/rendering_stats.cc
+++ b/cc/debug/rendering_stats.cc
@@ -64,6 +64,8 @@ void RenderingStats::EnumerateFields(Enumerator* enumerator) const {
                         total_image_gathering_time.InSecondsF());
   enumerator->AddDouble("totalTileAnalysisTimeInSeconds",
                         total_tile_analysis_time.InSecondsF());
+  enumerator->AddDouble("averageImplThreadToMainThreadTime",
+                        average_impl_thread_to_main_thread_time.InSecondsF());
 }
 
 void RenderingStats::Add(const RenderingStats& other) {
@@ -94,6 +96,8 @@ void RenderingStats::Add(const RenderingStats& other) {
   total_tiles_analyzed += other.total_tiles_analyzed;
   solid_color_tiles_analyzed += other.solid_color_tiles_analyzed;
   total_tile_analysis_time += other.total_tile_analysis_time;
+  average_impl_thread_to_main_thread_time +=
+      other.average_impl_thread_to_main_thread_time;
 }
 
 }  // namespace cc