Add FrameGenerator Unit 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 30b17f3da99bf188dcddf76cbfd9508f4e87a110..0cbb8262dcac07a4409c55c4746d426d382868a2 100644
--- a/services/ui/ws/frame_generator.cc
+++ b/services/ui/ws/frame_generator.cc
@@ -93,8 +93,10 @@ void FrameGenerator::SetExternalTilePriorityConstraints(
     const gfx::Transform& transform) {}
 
 void FrameGenerator::OnBeginFrame(const cc::BeginFrameArgs& begin_frame_args) {
-  if (!root_window_->visible())
+  if (!root_window_->visible() ||
+      begin_frame_args.type == cc::BeginFrameArgs::MISSED) {
     return;
+  }
 
   // TODO(fsamuel): We should add a trace for generating a top level frame.
   cc::CompositorFrame frame(GenerateCompositorFrame(root_window_->bounds()));
@@ -187,13 +189,13 @@ void FrameGenerator::SetNeedsBeginFrame(bool needs_begin_frame) {
     return;
 
   if (needs_begin_frame) {
-    begin_frame_source_->AddObserver(this);
     observing_begin_frames_ = true;

[Fady Samuel, 2017/03/17 14:06:40]

So I was looking at compositor_frame_sink_support.cc and I realized we are a bit
more succinct there. Perhaps this is nicer:

if (needs_begin_frame == observing_begin_frames_)
  return;

observing_begin_frames_ = needs_begin_frame;
if (needs_begin_frame)
  begin_frame_source_->AddObserver(this);
else
  begin_frame_source_->RemoveObserver(this);

[Alex Z., 2017/03/17 14:43:00]

Done.

+    begin_frame_source_->AddObserver(this);
     return;
   }
 
-  begin_frame_source_->RemoveObserver(this);
   observing_begin_frames_ = false;
+  begin_frame_source_->RemoveObserver(this);
 }
 
 }  // namespace ws