Pull up initFrameBuffer to ImageDecoder.

third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h

Index: third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h
diff --git a/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h b/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h
index d6169b58aeebc72104ed8a14f883ad644a14f644..448749834dc3de612cadf1e6d9245b192a01cd42 100644
--- a/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h
+++ b/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h
@@ -301,6 +301,12 @@ class PLATFORM_EXPORT ImageDecoder {
   // returns that number.
   virtual size_t decodeFrameCount() { return 1; }
 
+  // Called to initialize the frame buffer with the given index, based on the
+  // provided and previous frame's characteristics. Returns true on success. On
+  // failure, this will mark the image as failed. Before calling this method,
+  // the caller must verify that the frame exists.
+  bool initFrameBuffer(size_t);
+
   // Performs any additional setup of the requested frame after it has been
   // initially created, e.g. setting a duration or disposal method.
   virtual void initializeNewFrame(size_t) {}
@@ -341,6 +347,14 @@ class PLATFORM_EXPORT ImageDecoder {
     return total_size > ((1 << 29) - 1);
   }
 
+  // This methods gets called at the end of initFrameBuffer. Subclasses can do
+  // format specific initialization, for e.g. alpha settings, here.
+  virtual void onInitFrameBuffer(size_t){};
+
+  // Called by initFrameBuffer to determine if it can take the bitmap of the
+  // previous frame. This condition is different for GIF and WEBP.
+  virtual bool canReusePreviousFrameBuffer(size_t) const { return false; }

[scroggo_chromium, 2016/11/14 16:44:12]

I think this will only get called by WEBP, GIF, and PNG (once you add it), all
of which will override this. So a default implementation may not matter (we need
to pick one, since the others won't implement it), but my expectation would be
that the default returns true. (I guess false is safer, in some sense, so maybe
this is better as is.)

[joostouwerling, 2016/11/14 16:54:13]

I picked false for the last reason you mentioned. We could pick the GIF
condition since it is shared by both GIF and APNG but I think it is better to
explicitly state the condition in the specific decoder, with a comment
explaining why. 

+
   IntSize m_size;
   bool m_sizeAvailable = false;
   bool m_isAllDataReceived = false;