Rename GIFImageReader to SkGifImageReader

The former could violate One Definition Rule in Google3, since other
projects that are based on Chrome/webkit also have GIFImageReader.
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2445653004

Review-Url: https://codereview.chromium.org/2445653004
6 files changed