[tools] Specify affinity and raise priority when benchmarking

I plan to drive these options from the infra recipe.

Raising the priority of the benchmarked process, and fixing
its affinity to some other CPU than 0 improves stability of
measurements.

BUG=

Review-Url: https://codereview.chromium.org/1915303002
Cr-Commit-Position: refs/heads/master@{#35912}