This makes the compiler a few kilobytes smaller, and prepares getting
rid of TString altogether.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I93a003fe27b99bef72f872fa1066e2e108f934c5
Reviewed-on: https://chromium-review.googlesource.com/1107713Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>