SkDiscardablePixelRef objects are now marked immutable.

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
 
@@ skipping 19 matching lines @@
      *
      *  If non-NULL is returned, the caller is responsible for calling
      *  unref() on the data when it is finished.
      */
     virtual SkData* refEncodedData() { return NULL; }
 
     /**
      *  Return some information about the image, allowing the owner of
      *  this object to allocate pixels.
      *
+     *  Repeated calls to this function should give the same results,
+     *  allowing the PixelRef to be immutable.
+     *
      *  @return false if anything goes wrong.
      */
     virtual bool getInfo(SkImageInfo* info) = 0;
 
     /**
      *  Decode into the given pixels, a block of memory of size at
      *  least (info.fHeight - 1) * rowBytes + (info.fWidth *
      *  bytesPerPixel)
      *
+     *  Repeated calls to this function should give the same results,
+     *  allowing the PixelRef to be immutable.
+     *
      *  @param info A description of the format (config, size)
      *         expected by the caller.  This can simply be identical
      *         to the info returned by getInfo().
      *
      *         This contract also allows the caller to specify
      *         different output-configs, which the implementation can
      *         decide to support or not.
      *
      *  @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