[Background Fetch] Miscellaneous cleanups.

1. A check for state needs to be removed from a WPT test, since we've removed this
attribute from the BackgroundFetchRegistration object.
2. Update calls in matchImpl() from resolver->Reject to ScriptState::RejectWithDOMException.

Change-Id: Ie979ad3675ac7d093ae908604e683802db8115a0
Reviewed-on: https://chromium-review.googlesource.com/1230713
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592050}

background-fetch/fetch.https.window.js

@@ -139,7 +139,6 @@ backgroundFetchTest(async (test, backgroundFetch) => {
   assert_equals(results.length, 1);
 
   assert_equals(eventRegistration.id, registration.id);
-  assert_equals(eventRegistration.state, 'success');
   assert_equals(eventRegistration.failureReason, '');
 
   for (const result of results) {