Turn ASSERT_UNUSED into plain ASSERT

ASSERT_UNUSED used to be necessary, before the code paths for USE(CG)
were removed in https://codereview.chromium.org/13616008.

Review URL: https://codereview.chromium.org/288113009

git-svn-id: svn://svn.chromium.org/blink/trunk@174562 bbb929c8-8fbe-4397-9dbb-9b2b20218538