cc: Add TileManager interface that allows LTHI to determine when to activate pending tree.

cc/tile_priority.h

 // Copyright 2012 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.
 
 #ifndef CC_TILE_PRIORITY_H_
 #define CC_TILE_PRIORITY_H_
 
 #include <limits>
 
 #include "base/memory/ref_counted.h"
 #include "cc/picture_pile.h"
 #include "ui/gfx/rect.h"
 #include "ui/gfx/size.h"
 
 namespace cc {
 
 enum WhichTree {
   // Note: these must be 0 and 1 because we index with them in various places,
   // e.g. in Tile::priority_.
   ACTIVE_TREE = 0,
-  PENDING_TREE = 1
+  PENDING_TREE = 1,
+  NUM_TREES = 2
 };
 
 enum TileResolution {
   LOW_RESOLUTION = 0 ,
   HIGH_RESOLUTION = 1,
   NON_IDEAL_RESOLUTION = 2
 };
 
 struct CC_EXPORT TilePriority {
   TilePriority()
@@ skipping 72 matching lines @@
 
   size_t memory_limit_in_bytes;
 
   // Set when scrolling.
   bool smoothness_takes_priority;
 };
 
 }  // namespace cc
 
 #endif  // CC_TILE_PRIORITY_H_