Refactor TSymbol to follow naming conventions 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/793812 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>