Add buffer enqueue commands to front end and pass-through back end.

Bug: angleproject:6015
Change-Id: I936530d31903e395550e4540339ebec2e6702e65
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2928425
Commit-Queue: John Plate <jplate@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>