Mac: Don't repaint scrollbars every frame

cc/test/fake_layer_tree_host.cc

Index: cc/test/fake_layer_tree_host.cc
diff --git a/cc/test/fake_layer_tree_host.cc b/cc/test/fake_layer_tree_host.cc
index 0c4109e8d40e42bc53dfc28fa5953a487b91e6d5..741a1b4cf04fada27e732fe9efc3ba91595960d0 100644
--- a/cc/test/fake_layer_tree_host.cc
+++ b/cc/test/fake_layer_tree_host.cc
@@ -16,7 +16,8 @@ FakeLayerTreeHost::FakeLayerTreeHost(FakeLayerTreeHostClient* client,
                  &task_runner_provider_,
                  &manager_,
                  params->task_graph_runner),
-      needs_commit_(false) {
+      needs_commit_(false),
+      renderer_capabilities_set(false) {
   client_->SetLayerTreeHost(this);
 }
 
@@ -43,6 +44,12 @@ FakeLayerTreeHost::~FakeLayerTreeHost() {
   client_->SetLayerTreeHost(NULL);
 }
 
+const RendererCapabilities& FakeLayerTreeHost::GetRendererCapabilities() const {
+  if (renderer_capabilities_set)
+    return renderer_capabilities;
+  return LayerTreeHost::GetRendererCapabilities();
+}
+
 void FakeLayerTreeHost::SetNeedsCommit() { needs_commit_ = true; }
 
 LayerImpl* FakeLayerTreeHost::CommitAndCreateLayerImplTree() {