SetLowPriorityProcess's implementation can be shared between Mac and
Linux so even though it isn't using a Posix API, it lives in the posix
file.
CreateOSPixmap needed to be implemented but will never be used by dEQP
as our Mac EGL backend doesn't expose configs supporting pixmaps.

BUG=angleproject:891

Change-Id: I7b53b44b3465045954ec8e5997f703525ece92d0
Reviewed-on: https://chromium-review.googlesource.com/299851Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tryjob-Request: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>