Steps to reproduce the problem:
Test Case: https://thewebera.github.io/web-test-suite/web-nfc/nfc/nfc_watch.https.html
Test Name: Test that nfc.cancelWatch fails if there are no active watchers.
Code snippet:
return promise_rejects(test, "NotFoundError", navigator.nfc.cancelWatch())

What is the expected behavior?
NotFoundError is thrown.

What went wrong?
No error, test case time out.

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 60.0.3087.3  Channel: canary
OS Version: Nexus 7(Android 5.1)
Flash Version: Shockwave Flash 25.0 r0

According to spec: 10.8.4 The cancelWatch() method (https://w3c.github.io/web-nfc/#the-cancelwatch-method)
Description: 
If the parameter id is undefined, then remove all watches and filters set by successive calls of the NFC watch() method on all NFC adapters.
Otherwise, if the parameter id matches the local identifier of one of the previously set up watches, remove the corresponding watch.
  Otherwise, reject promise with "NotFoundError" and abort these steps.

So if there are no active watchers, "NotFoundError" should be thrown.