Fix GPU capture for Metal test apps.

Creating the next command buffer immediately seems to confuse
the Metal GPU capture command in Xcode. If the command buffer isn't
used until the next frame it apparently doesn't include it in the
capture. The solution is to delay creation until we first need it.

Change-Id: I95fea78a68356d319418ca15bd852e34d9efa12b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/329416
Reviewed-by: Adlai Holler <adlai@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed