Test that main thread frames cease to fire during composited animation

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 5bfd300482594c8f1690595ff918f1c1be176edc..f3f960ff96b51a774210d0280387865996faa551 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -306,6 +306,7 @@ LayerTreeHostImpl::~LayerTreeHostImpl() {
 void LayerTreeHostImpl::BeginMainFrameAborted(
     CommitEarlyOutReason reason,
     std::vector<std::unique_ptr<SwapPromise>> swap_promises) {
+  is_main_thread_frame_ = true;
   // If the begin frame data was handled, then scroll and scale set was applied
   // by the main thread, so the active tree needs to be updated as if these sent
   // values were applied and committed.
@@ -338,6 +339,7 @@ void LayerTreeHostImpl::BeginCommit() {
 
 void LayerTreeHostImpl::CommitComplete() {
   TRACE_EVENT0("cc", "LayerTreeHostImpl::CommitComplete");
+  is_main_thread_frame_ = true;
 
   UpdateSyncTreeAfterCommitOrImplSideInvalidation();
   micro_benchmark_controller_.DidCompleteCommit();
@@ -1581,6 +1583,7 @@ void LayerTreeHostImpl::OnCanDrawStateChangedForTree() {
 
 CompositorFrameMetadata LayerTreeHostImpl::MakeCompositorFrameMetadata() const {
   CompositorFrameMetadata metadata;
+  metadata.is_main_thread_frame = is_main_thread_frame_;
   metadata.device_scale_factor = active_tree_->painted_device_scale_factor() *
                                  active_tree_->device_scale_factor();
 
@@ -1760,6 +1763,8 @@ bool LayerTreeHostImpl::DrawLayers(FrameData* frame) {
   benchmark_instrumentation::IssueImplThreadRenderingStatsEvent(
       rendering_stats_instrumentation_->impl_thread_rendering_stats());
   rendering_stats_instrumentation_->AccumulateAndClearImplThreadStats();
+
+  is_main_thread_frame_ = false;
   return true;
 }