serialize NexusPlayer bots

Temporarily serialize the NexusPlayer Test bots to help diagnose
skia:7614.

Bug: skia:7614

Change-Id: I190327070f27e53d8344820f2215bad00f12e1ab
Reviewed-on: https://skia-review.googlesource.com/106621
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed