XSSAuditor false positive on large script blocks.

The snippet extraction logic is only valid for the start of a script block, therefore large scripts which may be passed to the auditor in multiple chunks must be processed in their entirety as was the first chunk.

BUG=302797
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/25726005

git-svn-id: svn://svn.chromium.org/blink/trunk@159172 bbb929c8-8fbe-4397-9dbb-9b2b20218538