Move ALLOW_UNUSED after identifiers.

This CL makes usage of ALLOW_UNUSED consistent with Chromium.

BUG=

Review URL: https://codereview.chromium.org/256733002

git-svn-id: svn://svn.chromium.org/blink/trunk@172720 bbb929c8-8fbe-4397-9dbb-9b2b20218538