cc: Disallow img.decode images from being checker imaged.

cc/tiles/tile_manager.h

Index: cc/tiles/tile_manager.h
diff --git a/cc/tiles/tile_manager.h b/cc/tiles/tile_manager.h
index 9b503e6af371d6875f86962721e9cf17a343cf15..86c66c4446efabb3b165138ebc0220b0ab5dca5b 100644
--- a/cc/tiles/tile_manager.h
+++ b/cc/tiles/tile_manager.h
@@ -243,6 +243,10 @@ class CC_EXPORT TileManager : CheckerImageTrackerClient {
     return num_of_tiles_with_checker_images_;
   }
 
+  CheckerImageTracker& checker_image_tracker() {
+    return checker_image_tracker_;
+  }
+
  protected:
   friend class Tile;
   // Must be called by tile during destruction.