cc: Simplify the logic for deciding to update tile priorities.

cc/layer_tree_impl.cc

Index: cc/layer_tree_impl.cc
diff --git a/cc/layer_tree_impl.cc b/cc/layer_tree_impl.cc
index 832ecaa84d1da7fe6abe6172d483d6215d2b4d67..088cf86363a40bd4fdddf08bcc4a3c319c325a3b 100644
--- a/cc/layer_tree_impl.cc
+++ b/cc/layer_tree_impl.cc
@@ -28,6 +28,7 @@ LayerTreeImpl::LayerTreeImpl(LayerTreeHostImpl* layer_tree_host_impl)
       scrolling_layer_id_from_previous_tree_(0),
       contents_textures_purged_(false),
       needs_update_draw_properties_(true),
+      needs_update_tile_priorities_(false),
       needs_full_tree_sync_(true) {
 }
 
@@ -213,13 +214,17 @@ struct UpdateTilePrioritiesForLayer {
 
 void LayerTreeImpl::UpdateDrawProperties(UpdateDrawPropertiesReason reason) {
   if (!needs_update_draw_properties_) {
-    if (reason == UPDATE_ACTIVE_TREE_FOR_DRAW && RootLayer())
+    if (needs_update_tile_priorities_ && RootLayer()) {
+      DCHECK_EQ(UPDATE_ACTIVE_TREE_FOR_DRAW, reason);
       LayerTreeHostCommon::callFunctionForSubtree<UpdateTilePrioritiesForLayer>(
           RootLayer());
+      needs_update_tile_priorities_ = false;
+    }
     return;
   }
 
   needs_update_draw_properties_ = false;
+  needs_update_tile_priorities_ = true;
   render_surface_layer_list_.clear();
 
   // For maxTextureSize.
@@ -249,6 +254,9 @@ void LayerTreeImpl::UpdateDrawProperties(UpdateDrawPropertiesReason reason) {
         settings().canUseLCDText,
         render_surface_layer_list_,
         update_tile_priorities);
+
+    if (update_tile_priorities)
+      needs_update_tile_priorities_ = false;
   }
 
   DCHECK(!needs_update_draw_properties_) <<