cc: Fix readback from non-root layers.

cc/test/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/pixel_test.h"
 
 #include "base/path_service.h"
 #include "base/run_loop.h"
 #include "cc/output/compositor_frame_metadata.h"
 #include "cc/output/gl_renderer.h"
@@ skipping 43 matching lines @@
 
 PixelTest::PixelTest()
     : device_viewport_size_(gfx::Size(200, 200)),
       fake_client_(new PixelTestRendererClient(device_viewport_size_)) {}
 
 PixelTest::~PixelTest() {}
 
 bool PixelTest::RunPixelTest(RenderPassList* pass_list,
                              const base::FilePath& ref_file,
                              const PixelComparator& comparator) {
+  return RunPixelTestWithReadbackTarget(pass_list,
+                                        pass_list->back(),
+                                        ref_file,
+                                        comparator);
+}
+
+bool PixelTest::RunPixelTestWithReadbackTarget(
+    RenderPassList* pass_list,
+    RenderPass* target,
+    const base::FilePath& ref_file,
+    const PixelComparator& comparator) {
   base::RunLoop run_loop;
 
-  pass_list->back()->copy_callbacks.push_back(
+  target->copy_callbacks.push_back(
       base::Bind(&PixelTest::ReadbackResult,
                  base::Unretained(this),
                  run_loop.QuitClosure()));
 
   renderer_->DecideRenderPassAllocationsForFrame(*pass_list);
   renderer_->DrawFrame(pass_list);
 
   // Wait for the readback to complete.
   resource_provider_->Finish();
   run_loop.Run();
@@ skipping 53 matching lines @@
   scoped_ptr<SoftwareOutputDevice> device(new SoftwareOutputDevice());
   output_surface_.reset(new OutputSurface(device.Pass()));
   resource_provider_ = ResourceProvider::Create(output_surface_.get(), 0);
   renderer_ = SoftwareRenderer::Create(
       fake_client_.get(),
       output_surface_.get(),
       resource_provider_.get()).PassAs<DirectRenderer>();
 }
 
 }  // namespace cc