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

services/ui/ws/frame_generator.cc

Index: services/ui/ws/frame_generator.cc
diff --git a/services/ui/ws/frame_generator.cc b/services/ui/ws/frame_generator.cc
index 87f0f1426288ac97294c4ec076df3ff790c908b7..ada61585a6b9f821081b5c9e9edc486f9abaec01 100644
--- a/services/ui/ws/frame_generator.cc
+++ b/services/ui/ws/frame_generator.cc
@@ -98,17 +98,24 @@ void FrameGenerator::SetExternalTilePriorityConstraints(
     const gfx::Transform& transform) {}
 
 void FrameGenerator::OnBeginFrame(const cc::BeginFrameArgs& begin_frame_args) {
+  current_begin_frame_ack_ = cc::BeginFrameAck(
+      begin_frame_args.source_id, begin_frame_args.sequence_number,
+      begin_frame_args.sequence_number, 0, false);

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

I'm unfamiliar, is it safe to mark this same sequence number as "positively
acknowledged (confirmed)" if we go through the early return just below?

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

At the moment, we don't intend to use the latest_confirmed_sequence_number for
FrameGenerator anywhere.

That said, it should be valid to set it to the acknowledged BeginFrame's
sequence number for two reasons:
1) If the window is invisible, we don't have updates to produce so can safely
confirm.
2) If the args are MISSED, we didn't have updates when the BeginFrame was
originally issued and thus it's fine to confirm it and include the updates in
the next one.

   if (!root_window_->visible() ||
       begin_frame_args.type == cc::BeginFrameArgs::MISSED) {
+    begin_frame_source_->DidFinishFrame(this, current_begin_frame_ack_);
     return;
   }
 
+  current_begin_frame_ack_.has_damage = true;
+  last_begin_frame_args_ = begin_frame_args;
+
   // TODO(fsamuel): We should add a trace for generating a top level frame.
   cc::CompositorFrame frame(GenerateCompositorFrame(root_window_->bounds()));
-
   compositor_frame_sink_->SubmitCompositorFrame(std::move(frame));
+
+  begin_frame_source_->DidFinishFrame(this, current_begin_frame_ack_);
   SetNeedsBeginFrame(false);
-  last_begin_frame_args_ = begin_frame_args;
 }
 
 const cc::BeginFrameArgs& FrameGenerator::LastUsedBeginFrameArgs() const {
@@ -150,6 +157,7 @@ cc::CompositorFrame FrameGenerator::GenerateCompositorFrame(
     frame.render_pass_list.push_back(std::move(invert_pass));
   }
   frame.metadata.device_scale_factor = device_scale_factor_;
+  frame.metadata.begin_frame_ack = current_begin_frame_ack_;
 
   if (window_manager_surface_info_.is_valid()) {
     frame.metadata.referenced_surfaces.push_back(