Disable extra CPUs during Nanobench

The previous experiment revealed that nanobench can
run on any of the online CPUs, so rather than put
the ones we don't need/want into powersave mode, just
disable them.

Maybe in the future we can run CPU tests on the big
or LITTLE cpus to get perf data on higher end or
lower end cpus, but only if we get very stable
results from this.

Bug: skia:7378
notry=true
notreechecks=true
Change-Id: I057513a691093e7f73c0f5790e17fab1a5ec0bc4
Reviewed-on: https://skia-review.googlesource.com/84820
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
22 files changed