Change SkDecodingImageGenerator API

src/lazy/SkDiscardablePixelRef.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 SkDiscardablePixelRef_DEFINED
 #define SkDiscardablePixelRef_DEFINED
 
 #include "SkDiscardableMemory.h"
 #include "SkImageGenerator.h"
 #include "SkImageInfo.h"
 #include "SkPixelRef.h"
 
+/**
+ *  A PixelRef backed by SkDiscardableMemory, with the ability to
+ *  re-generate the pixels (via a SkImageGenerator) if the DM is
+ *  purged.
+ *
+ *  Since SkColorTable is reference-counted, we do not support indexed
+ *  color with this class; there would be no way for the discardable
+ *  memory system to unref the color table.
+ */
 class SkDiscardablePixelRef : public SkPixelRef {
 public:
     SK_DECLARE_UNFLATTENABLE_OBJECT()
 
 protected:
     ~SkDiscardablePixelRef();
     virtual void* onLockPixels(SkColorTable**) SK_OVERRIDE;
     virtual void onUnlockPixels() SK_OVERRIDE;
     virtual bool onLockPixelsAreWritable() const SK_OVERRIDE { return false; }
 
@@ skipping 13 matching lines @@
     /* Takes ownership of SkImageGenerator. */
     SkDiscardablePixelRef(const SkImageInfo&, SkImageGenerator*,
                           size_t rowBytes,
                           SkDiscardableMemory::Factory* factory);
     friend bool SkInstallDiscardablePixelRef(SkImageGenerator*,
                                              SkBitmap*,
                                              SkDiscardableMemory::Factory*);
     typedef SkPixelRef INHERITED;
 };
 #endif  // SkDiscardablePixelRef_DEFINED