[cc] Plumb BeginFrameAcks through SurfaceManager to DisplayScheduler.

cc/surfaces/direct_compositor_frame_sink_unittest.cc

Index: cc/surfaces/direct_compositor_frame_sink_unittest.cc
diff --git a/cc/surfaces/direct_compositor_frame_sink_unittest.cc b/cc/surfaces/direct_compositor_frame_sink_unittest.cc
index ddea81e0b41837abf8e30b0f1bf17bf95d5823e1..f295ab0ca7f4dbb198906bf9e074f5837c869bb9 100644
--- a/cc/surfaces/direct_compositor_frame_sink_unittest.cc
+++ b/cc/surfaces/direct_compositor_frame_sink_unittest.cc
@@ -148,9 +148,5 @@ TEST_F(DirectCompositorFrameSinkTest, SuspendedDoesNotTriggerSwapBuffers) {
   EXPECT_EQ(2u, display_output_surface_->num_sent_frames());
 }
 
-// TODO(eseckler): Add back tests for BeginFrameAck forwarding through
-// DirectCompositorFrameSink and CompositorFrameSinkSupport when we add plumbing
-// of BeginFrameAcks through SurfaceObservers.
-
 }  // namespace
 }  // namespace cc