EGL_ANGLE_platform_angle_metal is the new extension name.

Bug: angleproject:2634
Change-Id: I6ebffbd4f6d7974bc5ef6d48849e985ee83e9808
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1848793Reviewed-by: Le Hoang Quyen <le.hoang.q@gmail.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>