Add a global core limit support for goma/rbe builds.

This is very usable for non Google-sized Goma clusters. We want to use
NINJA_CORE_MULTIPLIER logic, but at the same time we don't want to stall
our cluster with super-high -j values.

Bug: 1323475
Change-Id: I2ed414463b4f397a7bcebe42ea0b996e52006cda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3632492
Auto-Submit: Aleksey Khoroshilov <akhoroshilov@brave.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>

autoninja.py

@@ -176,9 +176,11 @@ def main(args):
 
       core_multiplier = int(
           os.environ.get('NINJA_CORE_MULTIPLIER', default_core_multiplier))
-
       j_value = num_cores * core_multiplier
 
+      core_limit = int(os.environ.get('NINJA_CORE_LIMIT', j_value))
+      j_value = min(j_value, core_limit)
+
       if sys.platform.startswith('win'):
         # On windows, j value higher than 1000 does not improve build
         # performance.