cros_run_unit_tests: limit parallelism to 10

BUG=chromium:627881
TEST=None

Change-Id: I6bd55795506beb5453d0ca7625f800d6825d03e2
Reviewed-on: https://chromium-review.googlesource.com/362995
Commit-Ready: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/scripts/cros_run_unit_tests.py b/scripts/cros_run_unit_tests.py
index b179121..a152bef 100644
--- a/scripts/cros_run_unit_tests.py
+++ b/scripts/cros_run_unit_tests.py
@@ -6,6 +6,7 @@
 
 from __future__ import print_function
 
+import multiprocessing
 import os
 
 from chromite.lib import commandline
@@ -97,7 +98,8 @@
     env = {'USE': use_flags}
 
   try:
-    chroot_util.RunUnittests(sysroot, pkg_with_test, extra_env=env)
+    chroot_util.RunUnittests(sysroot, pkg_with_test, extra_env=env,
+                             jobs=min(10, multiprocessing.cpu_count()))
   except cros_build_lib.RunCommandError:
     logging.error('Unittests failed.')
     raise