cc: Add a Scheduling.SwapAckWasFast UMA.

cc/scheduler/compositor_timing_history.h

Index: cc/scheduler/compositor_timing_history.h
diff --git a/cc/scheduler/compositor_timing_history.h b/cc/scheduler/compositor_timing_history.h
index 6ae539d5b5592a76b833083f2ab3b5c3ee640d85..c66eea41ac99149a34a9529a3c9d593e84dad073 100644
--- a/cc/scheduler/compositor_timing_history.h
+++ b/cc/scheduler/compositor_timing_history.h
@@ -117,6 +117,9 @@ class CC_EXPORT CompositorTimingHistory {
   base::TimeTicks draw_start_time_;
   base::TimeTicks swap_start_time_;
 
+  // Watchdog timers.
+  bool swap_ack_watchdog_enabled_;
+
   std::unique_ptr<UMAReporter> uma_reporter_;
   RenderingStatsInstrumentation* rendering_stats_instrumentation_;