Web Locks: Make 'signal' member non nullable in LockOptions

This CL makes 'signal' member in LockOption non nullable
to be spec conformant. [1]
It also drops a test that is inconsistent with this change.

[1] https://inexorabletash.github.io/web-locks/#idl-index


Bug: 855968
Change-Id: I4faf6582451347a05eeca34f02e25a43217d86cd
Reviewed-on: https://chromium-review.googlesource.com/1121953
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572244}

third_party/WebKit/LayoutTests/http/tests/locks/signal.html

@@ -25,14 +25,6 @@ promise_test(async t => {
         res, {signal}, t.unreached_func('callback should not run')),
       'Bindings should throw if the signal option is a not an AbortSignal');
   }
-
-  // These cases should work:
-  for (const signal of [null, undefined]) {
-    assert_equals(
-      undefined,
-      await navigator.locks.request(res, {signal}, lock => {}),
-      'Bindings should accept a null-ish signal option');
-  }
 }, 'The signal option must be an AbortSignal');
 
 promise_test(async t => {

third_party/blink/renderer/modules/locks/lock_options.idl

@@ -9,5 +9,5 @@ dictionary LockOptions {
     LockMode mode = "exclusive";
     boolean ifAvailable = false;
     boolean steal = false;
-    AbortSignal? signal;
+    AbortSignal signal;
 };