Checking if frame is complete and access duration doesn't need a decode

Source/core/platform/image-decoders/ImageDecoder.h

Index: Source/core/platform/image-decoders/ImageDecoder.h
diff --git a/Source/core/platform/image-decoders/ImageDecoder.h b/Source/core/platform/image-decoders/ImageDecoder.h
index 89ff44d9020644a02013366183e4ed7df77eef64..95f06b7ba627cc85222343923c7823ddc8bf7ca5 100644
--- a/Source/core/platform/image-decoders/ImageDecoder.h
+++ b/Source/core/platform/image-decoders/ImageDecoder.h
@@ -293,6 +293,13 @@ namespace WebCore {
         // Make the best effort guess to check if the requested frame has alpha channel.
         virtual bool frameHasAlphaAtIndex(size_t) const;
 
+        // Whether or not the frame is completely decoded or fully received.
+        // This method is used by animated images only.

[Peter Kasting, 2013/04/23 21:43:20]

I'm not terribly comfortable with this comment.  It may be currently true that
we only wind up calling this during animation (I'm not actually sure), but I
think the API of this class should support calling this for any image, meaning
we should provide a real implementation in the .cpp file.

Doing so should be easy and will greatly simplify the frameHasAlphaAtIndex()
implementation which basically implements the "frame complete?" check inline as
most of the function.

[Alpha Left Google, 2013/04/25 23:02:44]

I'm pretty sure this API is used only for animated images.

I agree with you that this API shouldn't be restricted to animated images. I'll
leave this as a TODO as simplifying frameHasAlphaAtIndex() and having a default
implementation for non-animated images is not a goal for this CL.

[Peter Kasting, 2013/04/25 23:22:32]

But you're adding the function in this CL, and the implementation that works for
both image types is like 2 lines long.  Why not do it now?  It's trivial.

+        virtual bool frameIsCompleteAtIndex(size_t) const { return false; }
+
+        // Duration for displaying a frame in seconds. This method is used by animated images only.
+        virtual float frameDurationAtIndex(size_t) const { return 0; }
+
         // Number of bytes in the decoded frame requested. Return 0 if not yet decoded.
         virtual unsigned frameBytesAtIndex(size_t) const;