Web Locks: If aborted due to an AbortSignal, use signal's reason

Updated in the spec a year ago[1], if the AbortSignal has a reason,
use that as the rejection, rather than a stock AbortError w/ hardcoded
message.

The WPTs didn't cover verifying the reason for an already-aborted
signal, so add coverage for those cases too.

[1] w3c/web-locks#86

Bug: 1279877
Change-Id: I9838805b10dbb3dae0371abe41faf922344a43f3

Author: inexorabletash

web-locks/signal.tentative.https.any.js

@@ -29,6 +29,36 @@ promise_test(async t => {
     'Request should reject with AbortError');
 }, 'Passing an already aborted signal aborts');
 
+promise_test(async t => {
+  const res = uniqueName(t);
+
+  const controller = new AbortController();
+  const reason = 'My dog ate it.';
+  controller.abort(reason);
+
+  const promise =
+        navigator.locks.request(res, {signal: controller.signal},
+                                t.unreached_func('callback should not run'));
+
+  await promise_rejects_exactly(
+    t, reason, promise, "Rejection should give the abort reason");
+}, 'Passing an already aborted signal rejects with the custom abort reason.');
+
+promise_test(async t => {
+  const res = uniqueName(t);
+
+  const controller = new AbortController();
+  controller.abort();
+
+  const promise =
+        navigator.locks.request(res, {signal: controller.signal},
+                                t.unreached_func('callback should not run'));
+
+  await promise_rejects_exactly(
+    t, controller.signal.reason, promise,
+    "Rejection should give the abort reason");
+}, 'Passing an already aborted signal rejects with the default abort reason.');
+
 promise_test(async t => {
   const res = uniqueName(t);