Prevent linear-time forcing of tokens by inducing XSSAuditor page blocks.

      The page itself must control where the fragment to match ends,
otherwise leading-substring matches may be induced.  The pre-conditions
required for this are expected to be uncommon.

BUG=498982

Review URL: https://codereview.chromium.org/1179633002

git-svn-id: svn://svn.chromium.org/blink/trunk@196971 bbb929c8-8fbe-4397-9dbb-9b2b20218538