Set correct symbol ids when referring to GLSL built-ins The symbol ids are fetched from the symbol table. A new utility function is added to make this more convenient. BUG=angleproject:1490 TEST=angle_unittests, angle_end2end_tests Change-Id: I780430e3386f6599503d8290c568ca9bc9cad147 Reviewed-on: https://chromium-review.googlesource.com/559535 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>