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..3af9569291bcb3415da8e62a9760f8166db50f10 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()));