Pre-rotation code were added to transform gl_Position, gl_FragCoords,
gl_PointCoords, dFdX, dFdY in shader. However, it is only useful for
android's surface pre-rotation and completely un-needed in Metal
back-end.

This CL disables these pre-rotation code if the platform is not
android.

Bug: angleproject:4678
Change-Id: I89c42fcf24b49896f4ed9c2f9465da521beaf25f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2295000
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Ian Elliott <ianelliott@google.com>