cc: Fix readback from non-root layers.

cc/test/layer_tree_pixel_test.h

 // Copyright 2013 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 "base/files/file_path.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "cc/layers/solid_color_layer.h"
 #include "cc/test/layer_tree_test.h"
 
@@ skipping 25 matching lines @@
                                                        SkColor color);
   scoped_refptr<SolidColorLayer> CreateSolidColorLayerWithBorder(
       gfx::Rect rect,
       SkColor color,
       int border_width,
       SkColor border_color);
 
   void RunPixelTest(scoped_refptr<Layer> content_root,
                     base::FilePath file_name);
 
+  void RunPixelTestWithReadbackTarget(scoped_refptr<Layer> content_root,
+                                      Layer* target,
+                                      base::FilePath file_name);
+
   // Common CSS colors defined for tests to use.
   enum Colors {
     kCSSOrange = 0xffffa500,
     kCSSBrown = 0xffa52a2a,
     kCSSGreen = 0xff008000,
   };
 
   scoped_ptr<PixelComparator> pixel_comparator_;
 
  private:
   scoped_refptr<Layer> content_root_;
+  Layer* readback_target_;
   base::FilePath ref_file_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TEST_LAYER_TREE_PIXEL_TEST_H_