cc: Add SwapAck watchdog.

cc/scheduler/compositor_timing_history.cc

Index: cc/scheduler/compositor_timing_history.cc
diff --git a/cc/scheduler/compositor_timing_history.cc b/cc/scheduler/compositor_timing_history.cc
index 8b474c09b3d20a070095c45f3c6cb3caf722bedd..c4267b0a5426a2f01bac1abf735104e5d5fd30a9 100644
--- a/cc/scheduler/compositor_timing_history.cc
+++ b/cc/scheduler/compositor_timing_history.cc
@@ -58,6 +58,16 @@ const double kPrepareTilesEstimationPercentile = 90.0;
 const double kActivateEstimationPercentile = 90.0;
 const double kDrawEstimationPercentile = 90.0;
 
+// The watchdog uses a combination of BeginImplFrame counts and
+// actual timestamps to avoid calling Now() too often or adding
+// unnecessary idle wakeups.
+// This will break if we aren't receiving BeginFrames, but this
+// is primarily for crbug.com/602486, where we are receiving them.
+const int kSwapAckWatchdogBeginImplFrameCount = 8 * 61;
+const int kSwapAckWatchdogBeginImplFrameReCount = 60;
+const base::TimeDelta kSwapAckWatchdogTimeout =
+    base::TimeDelta::FromMicroseconds(8000000);
+
 const int kUmaDurationMinMicros = 1;
 const int64_t kUmaDurationMaxMicros = base::Time::kMicrosecondsPerSecond / 5;
 const int kUmaDurationBucketCount = 100;
@@ -296,6 +306,7 @@ class NullUMAReporter : public CompositorTimingHistory::UMAReporter {
 
 CompositorTimingHistory::CompositorTimingHistory(
     bool using_synchronous_renderer_compositor,
+    bool enable_swap_ack_watchdog,
     UMACategory uma_category,
     RenderingStatsInstrumentation* rendering_stats_instrumentation)
     : using_synchronous_renderer_compositor_(
@@ -315,6 +326,8 @@ CompositorTimingHistory::CompositorTimingHistory(
       activate_duration_history_(kDurationHistorySize),
       draw_duration_history_(kDurationHistorySize),
       begin_main_frame_on_critical_path_(false),
+      swap_ack_watchdog_enabled_(enable_swap_ack_watchdog),
+      swap_ack_watchdog_begin_impl_frame_count_(0),
       uma_reporter_(CreateUMAReporter(uma_category)),
       rendering_stats_instrumentation_(rendering_stats_instrumentation) {}
 
@@ -443,6 +456,7 @@ void CompositorTimingHistory::DidCreateAndInitializeOutputSurface() {
   // After we get a new output surface, we won't get a spurious
   // swap ack from the old output surface.
   swap_start_time_ = base::TimeTicks();
+  swap_ack_watchdog_begin_impl_frame_count_ = 0;
 }
 
 void CompositorTimingHistory::WillBeginImplFrame(
@@ -458,6 +472,20 @@ void CompositorTimingHistory::WillBeginImplFrame(
     SetBeginMainFrameCommittingContinuously(false);
   }
 
+  // After N BeginImplFrames, check if the swap ack is taking too long.
+  // If it is, crash to get diagnostic information.
+  // Otherwise, check again in M BeginImplFrames.
+  if (swap_ack_watchdog_enabled_ && !swap_start_time_.is_null()) {
+    swap_ack_watchdog_begin_impl_frame_count_++;
+    if (swap_ack_watchdog_begin_impl_frame_count_ ==
+        kSwapAckWatchdogBeginImplFrameCount) {
+      base::TimeDelta swap_not_acked_time_ = Now() - swap_start_time_;
+      CHECK_LT(swap_not_acked_time_, kSwapAckWatchdogTimeout);
+      swap_ack_watchdog_begin_impl_frame_count_ -=
+          kSwapAckWatchdogBeginImplFrameReCount;
+    }
+  }
+
   did_send_begin_main_frame_ = false;
 }
 
@@ -697,6 +725,7 @@ void CompositorTimingHistory::DidDraw(bool used_new_active_tree,
 void CompositorTimingHistory::DidSwapBuffers() {
   DCHECK_EQ(base::TimeTicks(), swap_start_time_);
   swap_start_time_ = Now();
+  swap_ack_watchdog_begin_impl_frame_count_ = 0;
 }
 
 void CompositorTimingHistory::DidSwapBuffersComplete() {