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

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

Index: Source/core/platform/image-decoders/ImageDecoder.cpp
diff --git a/Source/core/platform/image-decoders/ImageDecoder.cpp b/Source/core/platform/image-decoders/ImageDecoder.cpp
index 7bbf39fc271f210770ecfcc5e73ea16ca6ba1907..901741eac4ae3636610f3317283997f11c8892e5 100644
--- a/Source/core/platform/image-decoders/ImageDecoder.cpp
+++ b/Source/core/platform/image-decoders/ImageDecoder.cpp
@@ -127,6 +127,11 @@ bool ImageDecoder::frameHasAlphaAtIndex(size_t index) const
     return true;
 }
 
+bool ImageDecoder::frameIsCompleteAtIndex(size_t) const
+{
+    return isAllDataReceived();

[Peter Kasting, 2013/04/27 00:51:01]

It seems like this should be:

    return ((index < m_frameBufferCache.size()) &&
(m_frameBufferCache[index].status() == ImageFrame::FrameComplete);

This will work better for e.g. ICO or other formats besides GIF that have
multiple frames.

Not including a trailing "|| isAllDataReceived()" part also lets us go ahead and
simplify frameHasAlphaAtIndex(), and is exactly equivalent to what the code did
before your patch, so it won't regress anything.  It will be slightly more
pessimistic in cases where we've gotten the data but hadn't decoded yet, but I
think it's actually safer.

The reason for this is that BitmapImage::cacheFrame() used to force us to decode
as much as possible, so the information we got was as accurate as possible, and
the "frame complete" flag would tell us if the information was reliable.  Now,
however, with anything other than GIF, cacheFrame() won't trigger any decoding,
and we'll store whatever the ImageDecoder base functions return for the various
values.  This can lead to us claiming a frame is complete when the other
metadata is inaccurate, which could cause problems.  I'm not sure whether it
_will_ cause problems as I haven't sufficiently audited all the codepaths, but
it definitely feels safer.

Long-term, if we want to support e.g. animated WebP, we'll want to do something
like you did for GIF and override these functions to be able to return accurate
answers while having only parsed the data, not decoded the framebuffer.  We also
may want to audit how the metadata is used more thoroughly, and perhaps plumb
the idea of "data available" versus "data parsed" versus "frame decoded" more
clearly to the ImageSource.  But I'd hope that can be a separate patch.

+}
+
 unsigned ImageDecoder::frameBytesAtIndex(size_t index) const
 {
     if (m_frameBufferCache.size() <= index)