ServiceWorker: Release a reference when it fails to dispatch ExtendableMessageEvent

This CL makes sure to release a reference to ServiceWorkerHandle that was
supposed to be sent to the destination.

Note: ExtendableMessageEvent is still behind a flag and resource leaks due to
this have not happened in release builds.

BUG=543198

Review URL: https://codereview.chromium.org/1799413002

Cr-Commit-Position: refs/heads/master@{#382455}
10 files changed