gclient is a .bat file on Windows, and subprocess won't run it unless shell=True.

BUG=angleproject:1204
TEST=run in Linux and Windows 8

Change-Id: I1f94673f23ef25fe35df64ffd1ad04ed69626ad0
Reviewed-on: https://chromium-review.googlesource.com/312324Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Austin Kinross <aukinros@microsoft.com>
Tested-by: Frank Henigman <fjhenigman@chromium.org>