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 f29f3da708421ee68854a4cdfaa2dd384e5e1b80..26dd97e3c9d414e118d8d14da821fdbd772b40b1 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -423,9 +423,9 @@ void LayerTreeHostImpl::PrepareTiles() {
   if (!tile_priorities_dirty_)
     return;
 
+  client_->WillPrepareTiles();
   tile_priorities_dirty_ = false;
   tile_manager_->PrepareTiles(global_tile_state_);
-
   client_->DidPrepareTiles();
 }