Clear VAO dirty bits in NULL back-end. This is the back-end responsibility now. Bug: chromium:921338 Change-Id: I6181210a80ec7d635382df74d842d8a899ad2894 Reviewed-on: https://chromium-review.googlesource.com/c/1409405 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>