Merge 147392 "Simply GIFImageReader error handling"

Merge 147392 "Simply GIFImageReader error handling"

> Simply GIFImageReader error handling
> https://bugs.webkit.org/show_bug.cgi?id=113718
> 
> Reviewed by Adam Barth.
> 
> When GIFImageReader encounters a parsing error it should just return
> without doing any more work. Current code saves a boolean before return,
> this is not necessary.
> 
> Tested with existing unit tests.
> Tested locally with ASAN build with 60k GIF images.
> 
> * platform/image-decoders/gif/GIFImageDecoder.cpp:
> (WebCore::GIFImageDecoder::repetitionCount):
> * platform/image-decoders/gif/GIFImageReader.cpp:
> (GIFImageReader::decode):
> * platform/image-decoders/gif/GIFImageReader.h:
> (GIFImageReader::GIFImageReader):
> (GIFImageReader):
> 

TBR=hclam@chromium.org