cc: Measure compositor timing with finer granularity

cc/trees/layer_tree_host_impl_unittest.cc

Index: cc/trees/layer_tree_host_impl_unittest.cc
diff --git a/cc/trees/layer_tree_host_impl_unittest.cc b/cc/trees/layer_tree_host_impl_unittest.cc
index f444fc339dd2c05bf2d4c7e4365d36b69bec4cd5..a52d9be0c718c8ee7dc83378b689e016428eb05a 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -148,6 +148,7 @@ class LayerTreeHostImplTest : public testing::Test,
     requested_animation_delay_ = delay;
   }
   void DidActivateSyncTree() override {}
+  void WillPrepareTiles() override {}
   void DidPrepareTiles() override {}
   void DidCompletePageScaleAnimationOnImplThread() override {
     did_complete_page_scale_animation_ = true;