Share ArrayString and Str helper methods. 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/206567 Reviewed-by: Nicolas Capens <capn@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>