This is to have the new "OS" for MacOS Sierra. Several types were
introduced in angle_config.h to minimize the angle-mods.patch to make
it easier to update in the future.

BUG=angleproject:2137
Change-Id: I7a98c5cb48d424f83eb39763eba0e51852a9b98f
Reviewed-on: https://chromium-review.googlesource.com/636202
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>