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The JS bindings return an array of results from a call to dispatch(),
containing the results returned by each listener. This is primarily
used, it seems, to detect whether or not a message port will be used
asynchronously through the onMessage event. Because of this, it is
(for now) necessary to also support in native bindings.

Add functionality to return the results from a JS dispatch() call on an
event. Since this is called directly from JS, we should know that
running JS synchronously from that point is safe.

Add tests for returning the result of dispatch() as well as end-to-end
tests for asynchronously responding to a message.

BUG=653596

Review-Url: https://codereview.chromium.org/2921013002
Cr-Commit-Position: refs/heads/master@{#478838}