cc: Record UMA for swap to swap ack latency.

cc/scheduler/scheduler.cc

Index: cc/scheduler/scheduler.cc
diff --git a/cc/scheduler/scheduler.cc b/cc/scheduler/scheduler.cc
index ecb54ba37e2b83209a8be0225d77aa5012dfb1fa..a8364cdd643f640b85c9288a55da0bb47349dfae 100644
--- a/cc/scheduler/scheduler.cc
+++ b/cc/scheduler/scheduler.cc
@@ -198,6 +198,7 @@ void Scheduler::SetMaxSwapsPending(int max) {
 }
 
 void Scheduler::DidSwapBuffers() {
+  compositor_timing_history_->DidSwapBuffers();
   state_machine_.DidSwapBuffers();
 
   // There is no need to call ProcessScheduledActions here because
@@ -209,6 +210,7 @@ void Scheduler::DidSwapBuffers() {
 
 void Scheduler::DidSwapBuffersComplete() {
   DCHECK_GT(state_machine_.pending_swaps(), 0) << AsValue()->ToString();
+  compositor_timing_history_->DidSwapBuffersComplete();
   state_machine_.DidSwapBuffersComplete();
   ProcessScheduledActions();
 }
@@ -260,6 +262,7 @@ void Scheduler::DidCreateAndInitializeOutputSurface() {
   TRACE_EVENT0("cc", "Scheduler::DidCreateAndInitializeOutputSurface");
   DCHECK(!frame_source_->NeedsBeginFrames());
   DCHECK(begin_impl_frame_deadline_task_.IsCancelled());
+  compositor_timing_history_->DidSwapBuffersReset();
   state_machine_.DidCreateAndInitializeOutputSurface();
   UpdateCompositorTimingHistoryRecordingEnabled();
   ProcessScheduledActions();