gfx: Rename canvas SaveAndUnscale to UnscaleTheDeviceScaleFactor.

ui/gfx/scoped_canvas.h

 // Copyright 2013 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_GFX_SCOPED_CANVAS_H_
 #define UI_GFX_SCOPED_CANVAS_H_
 
 #include "ui/gfx/canvas.h"
 
 namespace gfx {
 
 // Saves the drawing state, and restores the state when going out of scope.
 class ScopedCanvas {
  public:
   explicit ScopedCanvas(gfx::Canvas* canvas) : canvas_(canvas) {
     if (canvas_)
       canvas_->Save();
   }
   ~ScopedCanvas() {
     if (canvas_)
       canvas_->Restore();
   }
-  void SetCanvas(gfx::Canvas* canvas) {
-    if (canvas_)
-      canvas_->Restore();
-    canvas_ = canvas;
-    canvas_->Save();
-  }
 
  private:
   gfx::Canvas* canvas_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedCanvas);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_SCOPED_CANVAS_H_