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This change adds command-line option --stop-on-exit to IdleWakeups.
This makes IdleWakeups automatically stop measurement when the target
process closes, rather than needing to be stopped via Ctrl-C.

If the target process is not running when IdleWakeups starts, it runs
like normal until the target process is detected and then goes away,
at which point it stops.

Change-Id: Iaa11d79e13250c8887d7aacfbfde5d0696d76b08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2464442Reviewed-by: Alex Gough <ajgo@chromium.org>
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Cr-Commit-Position: refs/heads/master@{#817043}