Remove WebPInAcceptHeader flag from blink

This feature is now enabled, so removing the flag.

BUG=249474

Review URL: https://chromiumcodereview.appspot.com/17341007

git-svn-id: svn://svn.chromium.org/blink/trunk@152939 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add an opaque animated WebP image to Layout tests

This is to test the case when a frame rectangle doesn't contain alpha,
but the frame background does.

This in preparation for https://codereview.chromium.org/13980003/
The tests are skipped for now.

BUG=229641
R=noel@chromium.org

Review URL: https://codereview.chromium.org/17547002

git-svn-id: svn://svn.chromium.org/blink/trunk@152940 bbb929c8-8fbe-4397-9dbb-9b2b20218538