[ui/ws] Acknowledge BeginFrames in FrameGenerator + add tests.

services/ui/ws/frame_generator.h

Index: services/ui/ws/frame_generator.h
diff --git a/services/ui/ws/frame_generator.h b/services/ui/ws/frame_generator.h
index f9a0647dd0b88dce94e85e5050ad5f0931f06375..ea5887eaab1f8165024120430198ec5d2023a993 100644
--- a/services/ui/ws/frame_generator.h
+++ b/services/ui/ws/frame_generator.h
@@ -84,6 +84,7 @@ class FrameGenerator : public cc::CompositorFrameSinkClient,
 
   std::unique_ptr<cc::CompositorFrameSink> compositor_frame_sink_;
   cc::BeginFrameArgs last_begin_frame_args_;
+  cc::BeginFrameAck current_begin_frame_ack_;

[msw, 2017/03/21 17:38:57]

what's the difference between last and current?

[Eric Seckler, 2017/03/22 09:16:50]

This is args vs. ack. The ack is set and used (in GenerateCompositorFrame) while
the current BeginFrame is being processed (thus "current ack"). The args are
stored to implement the LastUsedBeginFrameArgs() interface of BeginFrameObserver
and are intended to be valid after the BeginFrame was completed (thus "last
args").

   cc::BeginFrameSource* begin_frame_source_ = nullptr;
   bool observing_begin_frames_ = false;
   bool high_contrast_mode_enabled_ = false;