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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: https://github.com/WICG/web-locks/issues/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 <jsbell@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601716}