Debugging WPT Waterfall timeouts.

Skipping the webdriver directory still caused timeouts. Instead, omit
the `wdspec` test type.

Bug: 1023835
Change-Id: Ied3971bf844f9ea9fb89f4f5c39cb2aeca01f904
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917690
Commit-Queue: Luke Z <lpz@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Auto-Submit: Luke Z <lpz@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715397}

testing/scripts/run_wpt_tests.py

@@ -75,7 +75,8 @@ class WPTTestAdapter(common.BaseIsolatedScriptArgsAdapter):
             # (crbug.com/1023835) The flags below are temporary to aid debugging
             "--log-mach=-",
             "--log-mach-verbose",
-            "--exclude=webdriver"
+            "--test-type=testharness",
+            "--test-type=reftest"
         ])
         return rest_args