[Cleanup] Avoid using deprecated v8::Function::New.

Moves to MaybeLocal version in api_event_handler.cc. This code should never through
when creating the function, so explicitly converting with ToLocalChecked().

BUG=v8:7290,v8:8238

Change-Id: I542384b457133b37d6279313bcabf874ad690591
Reviewed-on: https://chromium-review.googlesource.com/c/1349218
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610802}