More style cleanup in GIFImageReader

      https://bugs.webkit.org/show_bug.cgi?id=110776

Reviewed by Allan Sandfeld Jensen.

Renamed variable q to currentComponent which more accurately represent
the purpose.

No test. Simple refactoring.

* platform/image-decoders/gif/GIFImageReader.cpp:
(GIFImageReader::decodeInternal):

git-svn-id: svn://svn.chromium.org/blink/trunk@144257 bbb929c8-8fbe-4397-9dbb-9b2b20218538