Ensure that createImageBitmap takes a snapshot of a gifs frame 0 image.

BUG=255610, 166658

Review URL: https://chromiumcodereview.appspot.com/22346004

git-svn-id: svn://svn.chromium.org/blink/trunk@155785 bbb929c8-8fbe-4397-9dbb-9b2b20218538