cc: Measure compositor timing with finer granularity

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 c4a6373f51e46b0081c1a53b4f794d6ff974de5e..382ef7dbcc5318e25169505141c6f6ad75ea8285 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -416,9 +416,9 @@ void LayerTreeHostImpl::PrepareTiles() {
   if (!tile_priorities_dirty_)
     return;
 
+  client_->WillPrepareTiles();
   tile_priorities_dirty_ = false;
   tile_manager_->PrepareTiles(global_tile_state_);
-
   client_->DidPrepareTiles();
 }