views: Access the Canvas through recorders not through PaintContext.

ui/compositor/compositing_recorder.h

+// Copyright 2015 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.
+
+#ifndef UI_COMPOSITOR_COMPOSITING_RECORDER_H_
+#define UI_COMPOSITOR_COMPOSITING_RECORDER_H_
+
+#include "base/macros.h"
+#include "ui/compositor/compositor_export.h"
+
+namespace gfx {
+class Canvas;
+}
+
+namespace ui {
+class PaintContext;
+
+// A class to provide scoped compositing filters (eg opacity) of painting to a
+// DisplayItemList. The filters provided will be applied to any
+// DisplayItems added to the DisplayItemList while this object is alive. In
+// other words, any nested PaintRecorders or other such Recorders will
+// be filtered by the effect.
+class COMPOSITOR_EXPORT CompositingRecorder {
+ public:
+  CompositingRecorder(const PaintContext& context, float opacity);
+  ~CompositingRecorder();
+
+ private:
+  gfx::Canvas* canvas_;
+
+  DISALLOW_COPY_AND_ASSIGN(CompositingRecorder);
+};
+
+}  // namespace ui
+
+#endif  // UI_COMPOSITOR_CLIP_TRANSFORM_RECORDER_H_