Minor improvements to reftests.

sky/compositor/display_delegate_bitmap.cc

 // Copyright 2014 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 "sky/compositor/display_delegate_bitmap.h"
 
 #include "sky/compositor/layer_client.h"
 #include "third_party/skia/include/core/SkBitmapDevice.h"
 #include "third_party/skia/include/core/SkCanvas.h"
 #include "ui/gfx/codec/png_codec.h"
 #include "ui/gfx/geometry/rect.h"
 
 namespace sky {
 
 DisplayDelegateBitmap::DisplayDelegateBitmap(LayerClient* client)
   : client_(client) {}
 
 DisplayDelegateBitmap::~DisplayDelegateBitmap() {}
 
 DisplayDelegate* DisplayDelegateBitmap::create(LayerClient* client) {
   return new DisplayDelegateBitmap(client);
 }
 
 void DisplayDelegateBitmap::GetPixelsForTesting(std::vector<unsigned char>* pixels) {
-  gfx::PNGCodec::EncodeBGRASkBitmap(bitmap_, false, pixels);
+  bool success = gfx::PNGCodec::EncodeBGRASkBitmap(bitmap_, false, pixels);
+  CHECK(success) << "Could not dump pixels. Did you call notifyTestComplete before the first paint?";
 }
 
 void DisplayDelegateBitmap::Paint(mojo::GaneshSurface& surface, const gfx::Rect& size) {
   bitmap_.allocN32Pixels(size.width(), size.height());
   SkBitmapDevice device(bitmap_);
   SkCanvas canvas(&device);
   // Draw red so we can see when we fail to paint.
   canvas.drawColor(SK_ColorRED);
 
   client_->PaintContents(&canvas, size);
   canvas.flush();
 }
 
 }  // namespace sky