Web Locks API: Fix IDL glitch

A callback defined in Web IDL should take a nullable argument. We
don't enforce this in our bindings code and it can't be introspected
from JS so this is not a testable or web-facing change, just aligning
the code with a spec fix.

Spec issue: w3c/web-locks#51
Spec link: https://wicg.github.io/web-locks/#callbackdef-lockgrantedcallback

Change-Id: Icc2e076f62e45629a283d0039e2be30b4a6826c4
Reviewed-on: https://chromium-review.googlesource.com/c/1294421
Commit-Queue: Joshua Bell <[email protected]>
Commit-Queue: Victor Costan <[email protected]>
Reviewed-by: Victor Costan <[email protected]>
Cr-Commit-Position: refs/heads/master@{#601716}

web-locks/web-locks.idl

@@ -16,7 +16,7 @@ interface LockManager {
   Promise<LockManagerSnapshot> query();
 };
 
-callback LockGrantedCallback = Promise<any> (Lock lock);
+callback LockGrantedCallback = Promise<any> (Lock? lock);
 
 enum LockMode { "shared", "exclusive" };