This is a reland of 192da9d7

Re-land after OS upgrade to 10.15.

Original change's description:
> Use newer OSX SDK for iOS
>
> Based on http://crrev.com/c/2056465
>
> Bug: chromium:1052887
> Change-Id: I2f76067883ff528ca45becd90dd3a6f71a507494
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2059669
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Commit-Queue: Jamie Madill <jmadill@chromium.org>

Bug: chromium:1052887
Change-Id: If5d5c9e5b2dbb3b56eedfd9060015721817f47cd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2063040Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>