ui: Remove the canvas() member from PaintContext.

ui/compositor/paint_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/paint_recorder.h"
 
 #include "ui/compositor/paint_context.h"
 
 namespace ui {
 
 PaintRecorder::PaintRecorder(const PaintContext& context)
-    : canvas_(context.canvas()) {
+    : canvas_(context.canvas_) {
 }
 
 PaintRecorder::~PaintRecorder() {
 }
 
 }  // namespace ui