Set in_automation to true test_driver_internal.

This is in preparation for supporting manual interactions on testdriver.
By setting this field to true, unimplemented APIs will fail early rather
than waiting for user input (and timing out).

https://github.com/web-platform-tests/wpt/pull/11173

Change-Id: Ie409b683755358fb7978989bf3894e7b45b05239
Reviewed-on: https://chromium-review.googlesource.com/c/1352571
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611690}

third_party/blink/web_tests/resources/testdriver-vendor.js

@@ -138,4 +138,7 @@
     });
   };
 
+  // Enable automation so we don't wait for user input on unimplemented APIs
+  window.test_driver_internal.in_automation = true;
+
 })();