add peekPixels to canvas

src/core/SkDevice.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkDevice.h"
 #include "SkMetaData.h"
 
@@ skipping 99 matching lines @@
                                      srcRect.fLeft,
                                      srcRect.fTop,
                                      config8888);
     if (result && bmp == &tmp) {
         tmp.swap(*bitmap);
     }
     return result;
 }
 
 SkSurface* SkBaseDevice::newSurface(const SkImageInfo&) { return NULL; }
+
+const void* SkBaseDevice::peekPixels(SkImageInfo*, size_t*) { return NULL; }