Pull up initFrameBuffer to ImageDecoder.

third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.h

 /*
  * Copyright (C) 2006 Apple Computer, Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 64 matching lines @@
   void clearFrameBuffer(size_t frameIndex) override;
   virtual void decodeSize() { parse(GIFSizeQuery); }
   size_t decodeFrameCount() override;
   void initializeNewFrame(size_t) override;
   void decode(size_t) override;
 
   // Parses as much as is needed to answer the query, ignoring bitmap
   // data. If parsing fails, sets the "decode failure" flag.
   void parse(GIFParseQuery);
 
-  // Called to initialize the frame buffer with the given index, based on
-  // the previous frame's disposal method. Returns true on success. On
-  // failure, this will mark the image as failed.
-  bool initFrameBuffer(size_t frameIndex);
+  // Reset the alpha tracker for this frame. Before calling this method, the
+  // caller must verify that the frame exists.
+  void onInitFrameBuffer(size_t) override;
+
+  // When the disposal method of the frame is DisposeOverWritePrevious, the
+  // next frame will use the previous frame's buffer as its starting state, so
+  // we can't take over the data in that case. Before calling this method, the
+  // caller must verify that the frame exists.
+  bool canReusePreviousFrameBuffer(size_t) const override;
 
   bool m_currentBufferSawAlpha;
   mutable int m_repetitionCount;
   std::unique_ptr<GIFImageReader> m_reader;
 };
 
 }  // namespace blink
 
 #endif