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..1aa1b93c3ae2e61f757a6486f695e61293e709e5 100644
--- a/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h
+++ b/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.h
@@ -301,6 +301,16 @@ 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 callee must verify that the frame exists.

[scroggo_chromium, 2016/11/14 13:51:45]

caller*

[joostouwerling, 2016/11/14 16:25:09]

Done.

+  virtual bool initFrameBuffer(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 13:51:45]

This one doesn't actually need to be virtual. We never want to reuse for
DisposeOverwritePrevious or ImageFrame::BlendAtopPreviousFrame; it just so
happens that GIF doesn't use one, and WEBP doesn't use the other.

Once it's no longer virtual, it can be written in line or as a static method in
ImageDecoder.cpp

[joostouwerling, 2016/11/14 16:25:09]

For only GIF and WebP, this would be fine, but for APNG we can reuse
BlendAtopPreviousFrame. So with that it in mind, I think it is better to keep it
like this. 

I made it private though, since there is no need for subclasses to call this
method.

+
   // 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) {}