One of the mistakes one can make when running ninja is having goma
enabled (use_goma=true in args.gn) but not having goma running. This can
lead to ~1,000 failed compile steps, which is messy.

This change teaches autoninja.py to check whether goma is running. If
not then it tells autoninja to just print a warning message. The
check costs roughly 30 ms which seems reasonable.

In fact, because this change also switches away from vpython (necessary
to use python3 to use subprocess.run) it actually runs about 600 ms
_faster_ than before this change.

If build acceleration is requested through use_rbe then no checking for
whether the service is running is done. That could be added in the
future.

autoninja.py could auto-start goma but that is error prone and has
limited additional value.

This was tested on Linux, OSX, and Windows.

Bug: 868590, b/174673874
Change-Id: Ie773e574878471e5136b9b82d52f86af3d848318
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2627014
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>