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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}