Currently when user passes a CALayer to create WindowSurface, Metal
back-end will create a child CAMetalLayer to render to.

However, when this CALayer's contentsScale is changed, the
CAMetalLayer's contentsScale wasn't updated accordingly.

This CL fix this.

Bug: angleproject:2634
Change-Id: I20411aa8b81a30c6353e595680f1de00a4a24da7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2482407Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>