PerformanceTests/Parser/html-parser is only testing parsing of the head element

PerformanceTests/Parser/html-parser is only testing parsing of the head element
https://bugs.webkit.org/show_bug.cgi?id=69283

Reviewed by Adam Barth.

While investigating https://bugs.webkit.org/show_bug.cgi?id=68944
I found that the html-parser benchmark was only parsing up to the
first script tag per loop!  We've fixed this by adding the sandbox
tag which will deny all external loads and allow the parser to
synchronously continue parsing the entire document to completion
(as we had expected it was doing).

This changes the profile somewhat.  Line number counting is much
hotter, since we're actually accounting for the parse of the entire
document in our sample.

Total sample time only about doubles, from 1800ms to 3800ms on my machine
which is less than I would have expected.

* Parser/html-parser.html:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96531 268f45cc-cd09-0410-ab3c-d52691b4dbfc