Re-add tryserver.chromium.perf.json.

The file was removed in https://chromium-review.googlesource.com/c/569444/

But tryserver.chromium.perf is still using its custom code path, and not
the standard trybots.py that other tryservers use, so it still requires
tryserver.chromium.perf.json.

I'm in the process of converting it to trybots.py for Pinpoint, and we can
remove it later.

Bug: 742894
Change-Id: I062de1e561a36b186c20abb071bd5562578a31d1
Reviewed-on: https://chromium-review.googlesource.com/571858Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Annie Sullivan <sullivan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486861}

testing/buildbot/tryserver.chromium.perf.json

+{}