Implement missing util/ functions on Mac 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/299851 Reviewed-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>