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Redesigns the NamedMessagePortConnector system so that ports are
created in JS and pushed to native code, instead of being created on
the native side and pushed to JS.

The resulting implementation is cleaner because it allows callers to
use HTML5 MessagePorts directly, without any need for a buffered
wrapper class.

* Adapts JS callers to use MessagePort functions and events.
* Cleanup: modifies native dependents to take
  InterfaceHandle<MessagePort> instead of MessagePortPtrs.

Bug: 922718
Change-Id: I4ad7b381bd95e42859cd48abccee4fc8e4f64a75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1595175
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657367}