Using goma requires the developer to remember which build directories
use goma and which don't so that they can pass an appropriate -j number.
Getting this wrong makes builds slower, either by under utilizing
resources or by causing a self-inflicted DOS attack. Usage:

    autoninja -C out/debug

autoninja looks at the settings for the specified build directory and
then selects either -j num_cores*20 or no -j flag based on the
use_goma setting.

You can set the NINJA_CORE_MULTIPLIER variable to change from the
default 20* multiplier. You can also use NINJA_CORE_ADDITION if you
want non-goma builds to specify -j with an offset to the number of
cores, such as this Linux command:

    NINJA_CORE_ADDITION=-2 autoninja -C out/release base

This will tell autoninja to pass -j to ninja with num_cores-2 as the
parameter.

On Windows you can have a ninja.bat file (ahead of ninja on the path)
such that autoninja will automatically be used. It should contain this:
    @call autoninja.bat %*

Change-Id: I4003e3fc323d1cbab612999c945b5a8dc5bc6655
Reviewed-on: https://chromium-review.googlesource.com/517662Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Fumitoshi Ukai <ukai@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>