Implement eglWaitNative API. It will call XSync() on XCB and
do nothing on other systems.

Bug: angleproject:4281
Change-Id: I597b75124a380df519ab10af3cab9b6e26f3194f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2059620Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>