Add method to control whether local files get universal access during testing.

      This is required to test any form of local filesystem access restrictions.

BUG=20450
TEST=Needed for LayoutTests/fast/xmlhttprequest/xmlhttprequest-nonexistent-file.html

Review URL: http://codereview.chromium.org/384015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@31643 0039d316-1c4b-4281-b951-d872f2087c98