This will make it easier to make TStructure inherit from TSymbol.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I73a5a25234c7f965b6970891821ee9d6f0a23aca
Reviewed-on: https://chromium-review.googlesource.com/793812Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>