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 a17c656bbe87d3f724cdcd38d6e2ca150027c3e6..225745111a2c00faa483c564b343f9c53e03e401 100644
--- a/cc/scheduler/scheduler.cc
+++ b/cc/scheduler/scheduler.cc
@@ -201,6 +201,7 @@ void Scheduler::SetMaxSwapsPending(int max) {
 }
 
 void Scheduler::DidSwapBuffers() {
+  compositor_timing_history_->DidSwapBuffers();
   state_machine_.DidSwapBuffers();
 
   // There is no need to call ProcessScheduledActions here because
@@ -212,6 +213,7 @@ void Scheduler::DidSwapBuffers() {
 
 void Scheduler::DidSwapBuffersComplete() {
   DCHECK_GT(state_machine_.pending_swaps(), 0) << AsValue()->ToString();
+  compositor_timing_history_->DidSwapBuffersComplete();
   state_machine_.DidSwapBuffersComplete();
   ProcessScheduledActions();
 }
@@ -261,6 +263,7 @@ 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();