cc: Fix readback from non-root layers.

cc/test/layer_tree_pixel_test.cc

 // 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 "cc/test/layer_tree_pixel_test.h"
 
 #include "base/path_service.h"
 #include "cc/test/paths.h"
 #include "cc/test/pixel_comparator.h"
 #include "cc/test/pixel_test_utils.h"
@@ skipping 45 matching lines @@
   // To rebaseline:
   // EXPECT_TRUE(WritePNGFile(*bitmap, test_data_dir.Append(ref_file_), true));
 
   EXPECT_TRUE(MatchesPNGFile(*bitmap,
                              test_data_dir.Append(ref_file_),
                              *pixel_comparator_));
   EndTest();
 }
 
 void LayerTreePixelTest::BeginTest() {
-  layer_tree_host()->root_layer()->RequestCopyAsBitmap(
+  Layer* target = readback_target_ ? readback_target_
+                                   : layer_tree_host()->root_layer();
+  target->RequestCopyAsBitmap(
       base::Bind(&LayerTreePixelTest::ReadbackResult,
                  base::Unretained(this)));
   PostSetNeedsCommitToMainThread();
 }
 
 void LayerTreePixelTest::AfterTest() {}
 
 scoped_refptr<SolidColorLayer> LayerTreePixelTest::CreateSolidColorLayer(
     gfx::Rect rect, SkColor color) {
   scoped_refptr<SolidColorLayer> layer = SolidColorLayer::Create();
@@ skipping 30 matching lines @@
   layer->AddChild(border_left);
   layer->AddChild(border_right);
   layer->AddChild(border_bottom);
   return layer;
 }
 
 void LayerTreePixelTest::RunPixelTest(
     scoped_refptr<Layer> content_root,
     base::FilePath file_name) {
   content_root_ = content_root;
+  readback_target_ = NULL;
   ref_file_ = file_name;
   RunTest(true);
 }
+
+void LayerTreePixelTest::RunPixelTestWithReadbackTarget(
+    scoped_refptr<Layer> content_root,
+    Layer* target,
+    base::FilePath file_name) {
+  content_root_ = content_root;
+  readback_target_ = target;
+  ref_file_ = file_name;
+  RunTest(true);
+}
 
 void LayerTreePixelTest::SetupTree() {
   scoped_refptr<Layer> root = Layer::Create();
   root->SetBounds(content_root_->bounds());
   root->AddChild(content_root_);
   layer_tree_host()->SetRootLayer(root);
   LayerTreeTest::SetupTree();
 }
 
 }  // namespace cc