views: Access the Canvas through recorders not through PaintContext.

ui/compositor/compositing_recorder.cc

+// 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.
+
+#include "ui/compositor/compositing_recorder.h"
+
+#include "ui/compositor/paint_context.h"
+#include "ui/gfx/canvas.h"
+
+namespace ui {
+
+CompositingRecorder::CompositingRecorder(const PaintContext& context,
+                                         float opacity)
+    : canvas_(context.canvas()) {
+  canvas_->SaveLayerAlpha(0xff * opacity);
+}
+
+CompositingRecorder::~CompositingRecorder() {
+  canvas_->Restore();
+}
+
+}  // namespace ui