Looping of animated WebP: Loop count off-by-one

Animation system cycles animated images repetitionCount() + 1 times.

So, need to set repetitionCount to one less than actual.

BUG=

Review URL: https://codereview.chromium.org/284043003

git-svn-id: svn://svn.chromium.org/blink/trunk@174323 bbb929c8-8fbe-4397-9dbb-9b2b20218538