Check if priority actually changed in Tile::SetPriority()

cc/resources/tile.cc

Index: cc/resources/tile.cc
diff --git a/cc/resources/tile.cc b/cc/resources/tile.cc
index 48f3ae78ebd0600d04ab0f8ecce7d03f9389768d..f29baf6907ac83b00cecfa0a54afc56de0bc4fb1 100644
--- a/cc/resources/tile.cc
+++ b/cc/resources/tile.cc
@@ -46,6 +46,9 @@ scoped_ptr<base::Value> Tile::AsValue() const {
 }
 
 void Tile::SetPriority(WhichTree tree, const TilePriority& priority) {
+  if (priority_[tree] == priority)
+    return;
+
   tile_manager_->WillModifyTilePriority(this, tree, priority);
   priority_[tree] = priority;
 }