Respect default goma path on Windows (#7643)

tools/gn

@@ -199,6 +199,11 @@ def to_gn_args(args):
 
     goma_dir = os.environ.get('GOMA_DIR')
     goma_home_dir = os.path.join(os.getenv('HOME', ''), 'goma')
+
+    # GOMA has a different default (home) path on gWindows.
+    if not os.path.exists(goma_home_dir) and sys.platform.startswith(('cygwin', 'win')):
+      goma_home_dir = os.path.join('c:\\', 'src', 'goma', 'goma-win64')
+
     if args.goma and goma_dir:
       gn_args['use_goma'] = True
       gn_args['goma_dir'] = goma_dir