GL: Refactor ShaderGL to not hold renderer objects. BUG=angleproject:2464 Change-Id: I243c010e30d62e233411b1dab1d13a69b44e4b38 Reviewed-on: https://chromium-review.googlesource.com/1054214 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>