Currently, the most time consuming operation when creating a new browser
during Telemetry tests consists of sending an "adb force-stop" command
to the device, in order to force kill any previous browsers which did
not close as expected.
Sending a kill -9 to the process is, however, more efficient.
This CL also adds some efficiency improvements to DeviceUtils.KillAll.
The change amounts to a 6-7% speedup when running all of telemetry
unittests.
BUG=379378
Review URL: https://codereview.chromium.org/
536343003
Cr-Commit-Position: refs/heads/master@{#294368}