This is another corner case where clutter from the page may prevent the
XSSAuditor from performing a proper match. Compare attr="subvalue;subvalue;clutter"
on a per-subvalue basis for the first suspicious subvalue.

BUG=384077

Review URL: https://codereview.chromium.org/337143004

git-svn-id: svn://svn.chromium.org/blink/trunk@176359 bbb929c8-8fbe-4397-9dbb-9b2b20218538