Index symbols by id in ValidateOutputs This is cleaner than indexing them by their name string. BUG=angleproject:2267 TEST=angle_unittests Change-Id: I6cdc4125fadb6c7049ad8664e8a66da2a9a5bb8d Reviewed-on: https://chromium-review.googlesource.com/836892 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>