Enable deferred image decoding for GIF images

A better benchmark measurement is in the tree this feature is safe to
turn on.

BUG=259783,169377,260052
TBR=senorblanco@chromium.org

Review URL: https://codereview.chromium.org/20829002

git-svn-id: svn://svn.chromium.org/blink/trunk@155022 bbb929c8-8fbe-4397-9dbb-9b2b20218538