Force locale to en-US for TimeFormat tests.

This allows the tests to pass in non-english locales.

BUG=none
TEST=improves stability of existing tests

Review URL: https://codereview.chromium.org/150503002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248690 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248690