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.

Reland with a Linux compilation fix (forgot to delete
SetLowPriorityProcess).

BUG=angleproject:891

Change-Id: If8145501e1787d08d82402a6baa60a5404cac6b1
Reviewed-on: https://chromium-review.googlesource.com/299773Tested-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>