Bug 1509603 [wpt PR 14217] - Set default for manifest_update flag appropriately., a=testonly

Automatic update from web-platform-tests
Set default for manifest_update flag appropriately.
--

wpt-commits: 81607985fc9485cbf9f3d0a8ae74359f885eb36b
wpt-pr: 14217

testing/web-platform/tests/tools/wptrunner/wptrunner/wptcommandline.py

@@ -47,7 +47,7 @@ def create_parser(product_choices=None):
 
 TEST is either the full path to a test file to run, or the URL of a test excluding
 scheme host and port.""")
-    parser.add_argument("--manifest-update", action="store_true", default=True,
+    parser.add_argument("--manifest-update", action="store_true", default=None,
                         help="Regenerate the test manifest.")
     parser.add_argument("--no-manifest-update", action="store_false", dest="manifest_update",
                         help="Prevent regeneration of the test manifest.")
@@ -434,6 +434,9 @@ def check_args(kwargs):
     if kwargs["product"] is None:
         kwargs["product"] = "firefox"
 
+    if kwargs["manifest_update"] is None:
+        kwargs["manifest_update"] = True
+
     if "sauce" in kwargs["product"]:
         kwargs["pause_after_test"] = False