Fix handling of broken GIFs with weird frame sizes

      Code didn't handle well if a GIF frame has dimension greater than the
&quotscreen&quot dimension. This will break deferred image decoding.

This change reports the size as final only when the first frame is
encountered.

Added a test to verify this behavior. Frame size reported by the decoder
should be constant.

BUG=437651
R=pkasting@chromium.org, senorblanco@chromium.org

Review URL: https://codereview.chromium.org/813943003

git-svn-id: svn://svn.chromium.org/blink/trunk@188423 bbb929c8-8fbe-4397-9dbb-9b2b20218538