Properly check for debug and robust access context creation flags. These flags can come from both attributes and bitfield flags. Also move the robust access check to an UNIMPLEMENTED in Context, dEQP ES 3.1 tests attempt to use this flag and it's convenient to be able test by disabling UNIMPLEMENTED assertions. BUG=angleproject:1442 Change-Id: I1641fdb3afee308235351ebd60be680765b16b3b Reviewed-on: https://chromium-review.googlesource.com/405774 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>