Reland "Minimal iOS app: Perform present subsequent to flush"

This is a reland of 4e385e21a41fe7a7a9c8156d9a5a0d48f31ab64a

Original change's description:
> Minimal iOS app: Perform present subsequent to flush
> 
> Currently the backbuffer present is not on the same MTLCommandQueue
> as the flush from Skia, which probably means that it's happening
> concurrently. This can overwhelm the GPU. This CL submits the present
> to the same queue as the flush.
> 
> Change-Id: Ibaf805553931c9dc46368b362a2391425ae3e60e
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/248557
> Reviewed-by: Hal Canary <halcanary@skia.org>
> Commit-Queue: Jim Van Verth <jvanverth@google.com>

Change-Id: I118047630b8936838d5da8ef4307ad5c5e8d2ade
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/248562
Reviewed-by: Hal Canary <halcanary@skia.org>
Commit-Queue: Jim Van Verth <jvanverth@google.com>