TString was being used in some places where it was not really needed.
Clean these up.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: Ib7fd26f9c6b6b885433c840a9520393908f1f902
Reviewed-on: https://chromium-review.googlesource.com/887068Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>