Change SkDecodingImageGenerator API

include/core/SkImageGenerator.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkImageGenerator_DEFINED
 #define SkImageGenerator_DEFINED
 
 #include "SkDiscardableMemory.h"
 #include "SkImageInfo.h"
 
 class SkBitmap;
 class SkData;
 class SkImageGenerator;
 
 /**
  *  Takes ownership of SkImageGenerator.  If this method fails for
  *  whatever reason, it will return false and immediatetely delete
  *  the generator.  If it succeeds, it will modify destination
  *  bitmap.
  *
+ *  If generator is NULL, will safely return false.
+ *
  *  If this fails or when the SkDiscardablePixelRef that is
  *  installed into destination is destroyed, it will call
  *  SkDELETE() on the generator.  Therefore, generator should be
  *  allocated with SkNEW() or SkNEW_ARGS().
  *
  *  @param destination Upon success, this bitmap will be
  *  configured and have a pixelref installed.
  *
  *  @param factory If not NULL, this object will be used as a
  *  source of discardable memory when decoding.  If NULL, then
@@ skipping 56 matching lines @@
      *
      *  @return false if anything goes wrong or if the image info is
      *          unsupported.
      */
     virtual bool getPixels(const SkImageInfo& info,
                            void* pixels,
                            size_t rowBytes) = 0;
 };
 
 #endif  // SkImageGenerator_DEFINED