Always allow file:// URLs.

Once a file is on the local drive, there's no point in trying to filter access
to it. This is especially applicable to Chrome OS, where downloaded files
cannot be viewed otherwise.

BUG=337649
TEST=unit_tests

Review URL: https://codereview.chromium.org/135723015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249457 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249457