The correct metal layer's drawable size should be calculated after
mMetalLayer.get().contentsScale is set during initialization to allow
initial viewport to be computed properly on front-end side.

Bug: angleproject:2634
Change-Id: If278f40c0b6800bbe12fb5ea94986f728381bc00
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2687300
Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>