Reviewed by Kent Tamura.

        [Chromium/DRT] Empty out user stylesheet after each test run.
        https://bugs.webkit.org/show_bug.cgi?id=45282

        This should significantly cut down on the number of the mysterious flaky tests
        whose diffs looked like the page was blown up to a very lage size. This
        was indeed the dirty work of platform/mac/fast/loader/user-stylesheet-fast-path.html,
        which set the base body font to 100px.

        Since the user stylesheet was never reset, _all_ pixel tests that ran after it in
        the same thread failed.

        * DumpRenderTree/chromium/TestShell.cpp:
        (TestShell::resetWebSettings): Set user stylesheet to an empty URL.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@66861 268f45cc-cd09-0410-ab3c-d52691b4dbfc