[infra] Prepare gn.py for update to GN binary

First part in attempt to reland https://codereview.chromium.org/2854583002/
in two parts. If the second part fails, this part does not need to be
reverted.

Review-Url: https://codereview.chromium.org/2859833003 .

tools/gn.py

@@ -367,7 +367,8 @@ def parse_args(args):
       type=int,
       help='Number of simultaneous GN invocations',
       dest='workers',
-      default=multiprocessing.cpu_count())
+      # Set to multiprocessing.cpu_count() when GN can be run in parallel.
+      default=1)
 
   options = parser.parse_args(args)
   if not process_options(options):