Tweaked the gif2webp_util API to support this. Requested in: https://code.google.com/p/webp/issues/detail?id=144 (cherry picked from commit 65e5eb8a62923424f3d1142a811f0e42341f161d) Change-Id: I0e8c4edc39227355cd8d3acc55795186e25d0c3a