Add some string helper classes. These helper functions mirror some utilities in Chrome, for splitting strings along whitespace, or reading file contents into a string. Also remove the hack for skipping the doubly-defined GLX header. BUG=angleproject:892,angleproject:998 Change-Id: Ie617be7e02c2ccc684028500aabb54561c9c6c14 Reviewed-on: https://chromium-review.googlesource.com/272436 Reviewed-by: Brandon Jones <bajones@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>