This makes the TSymbolTable interface cleaner and prepares for making
unique id counting thread-safe.

BUG=angleproject:624
TEST=angle_unittests

Change-Id: Ief99c9fc777603de28ba1517e351bc8a00633590
Reviewed-on: https://chromium-review.googlesource.com/570418
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>