After http://trac.webkit.org/changeset/151957 frameDurationAtIndex() and frameIsCompleteAtIndex() functions don't ensure decoding the image. Unfortunaly ImageSourceCG.cpp still expects is and it caused a nasty bug on Mac: https://bugs.webkit.org/show_bug.cgi?id=119016
It blocks fixing flakey crashes during animated gifs on all ports except CG ports: https://bugs.webkit.org/show_bug.cgi?id=116041
I changed my mind, after rolling out the original fix, I'm going to fix this issue in the original bug: https://bugs.webkit.org/show_bug.cgi?id=116041