Remove the use of WebPInAcceptHeader flag from Blink

BUG=249474

Review URL: https://chromiumcodereview.appspot.com/16917012

git-svn-id: svn://svn.chromium.org/blink/trunk@152915 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Suppress some flaky worker tests (surprise).

TBR=dpranke
BUG=

Review URL: https://codereview.chromium.org/17379028

git-svn-id: svn://svn.chromium.org/blink/trunk@152916 bbb929c8-8fbe-4397-9dbb-9b2b20218538