Refactor the PlatformContext layer to have only one class.

skia/ext/platform_canvas.cc

+// Copyright (c) 2006-2008 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 "skia/ext/platform_canvas.h"
+#include "third_party/skia/include/core/SkTypes.h"
+
+namespace skia {
+
+SkDevice* PlatformCanvas::setBitmapDevice(const SkBitmap&) {
+  SkASSERT(false);  // Should not be called.
+  return NULL;
+}
+
+PlatformDevice& PlatformCanvas::getTopPlatformDevice() const {
+  // All of our devices should be our special PlatformDevice.
+  SkCanvas::LayerIter iter(const_cast<PlatformCanvas*>(this), false);
+  return *static_cast<PlatformDevice*>(iter.device());
+}
+
+// static
+size_t PlatformCanvas::StrideForWidth(unsigned width) {
+  return 4 * width;
+}
+
+}  // namespace skia