This is now required for upcoming buffer mapping resets.

Test: Angry Birds 2 MEC
Bug: b/157672184
Change-Id: I6358acf42ca9b92bfff621f12f207be7c3655e52
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2231801Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>