This reverts commit 428143ee.

Reason for revert: Fixing the issues revealed by the original change by
avoiding python3 and by checking for the existence of gomacc[.exe]
before running it.

This also relands 2241db8a - "Avoid
capture_output to support Python 3.6", to simplify relanding and any
possible reverts.

Original change's description:
> Revert "Check whether goma is running when it is enabled"
>
> This reverts commit b7ddc5a0.
>
> Reason for revert:
> This broke the builder where depot_tools is not in PATH.
> https://logs.chromium.org/logs/infra-internal/buildbucket/cr-buildbucket.appspot.com/8858077852309878080/+/u/build/stdout
>
> Original change's description:
> > Check whether goma is running when it is enabled
> >
> > 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>
>
> TBR=yyanagisawa@google.com,dpranke@google.com,brucedawson@chromium.org,sanfin@chromium.org,infra-scoped@luci-project-accounts.iam.gserviceaccount.com
>
> Change-Id: I57a6c73ea853259f3d1ec7ad0ce51e495acc96db
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 868590
> Bug: b/174673874
> Bug: 1167064
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2632018
> Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
> Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@google.com>

TBR=yyanagisawa@google.com,dpranke@google.com,brucedawson@chromium.org,sanfin@chromium.org,infra-scoped@luci-project-accounts.iam.gserviceaccount.com

# Not skipping CQ checks because this is a reland.

Bug: 868590
Bug: b/174673874
Bug: 1167064
Change-Id: I8aa6830259bc18f8e7926cd0bf5c62e671c74a2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2634201Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>