cc: Separate the priority from the tile and put in new PrioritizedTile

cc/resources/prioritized_tile.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "cc/resources/prioritized_tile.h"
+
+#include "cc/resources/picture_layer_tiling.h"
+
+namespace cc {
+
+PrioritizedTile::PrioritizedTile() : tile_(nullptr), is_occluded_(false) {
+}
+
+PrioritizedTile::~PrioritizedTile() {
+}
+
+void PrioritizedTile::UpdateTile(Tile* tile, PictureLayerTiling* tiling) {
+  tile_ = tile;
+  if (tile_)
+    tiling->UpdateTilePriority(this);
+}
+
+}  // namespace cc