It is sometimes helpful to do a full build of Chrome using goma while
online and then do incremental builds without goma while offline or with
poor network bandwidth. Turning off goma in gn args triggers a full
rebuild but the GOMA_DISABLED environment variable can be used to
disable goma without triggering a rebuild.

In order to make this feature easier to discover and use this change
adds support for -o/--offline to autoninja. autoninja -h will mention
this flag and autoninja.bat/autoninja.py handle it appropriately. This
means setting the environment variable in autoninja.bat, and using it
to adjust the -j value and stripping it out in autoninja.py.

The bash script that wraps autoninja.py on Linux and OSX did not need
updating because the Python script can emit 'GOMA_DISABLED=1 ninja ...'
and this is executed by bash.

This was produced during pair programming with jessemckenna@

Bug: b/172039612
Change-Id: Ifcfbc598ac20f23e5fe013e02979b5b8c2851b01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2510818
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>