Skip ProcessMemoryMetricsEmitterTest on official builders

Official builders don't use fieldtrial_testing_config.json, so they
don't get the same feature flags as most other bots.

Bug: 1284935
Change-Id: Id81d3348705b9abe1c382b61a697344295084f57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3383682


Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Cr-Commit-Position: refs/heads/main@{#958208}