This implements glCreateMemoryObjectsEXT, glDeleteMemoryObjectsEXT, and
glIsMemoryObjectEXT. It's not possible to do anything useful with them
yet.

Bug: angleproject:3289

Change-Id: I8882b657e9de564b5f97f8dea87838f67b1928f8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1552025
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>