Placing these string helper methods allows us to re-use them in
the relevant place of the shader translator.

BUG=angle:466

Change-Id: Idd638542027d3b1035bc79fc941e80bf436c82af
Reviewed-on: https://chromium-review.googlesource.com/206567Reviewed-by: Nicolas Capens <capn@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>