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Lays groundwork for associated interfaces in Mojo JS modules. The
overall theme here is to introduce a new internal Endpoint abstraction
to back each remote and receiver. Each Endpoint is in turn backed by a
new internal Router abstraction which models shared ownership of a
message pipe handle.

Router multiplexes the pipe by routing incoming messages to a specific
Endpoint based on the message header's interface ID, and can also
send messages on each Endpoint's behalf.

JS (lite) bindings are refactored here to be built on these
abstractions, with no interesting behavioral changes intended.

Bug: 914165, 1004256
Change-Id: I3c9aef111ccb5a285124d1f94f3c45f40aa8d7ae
Test: covered by many WebUI browser tests and Blink web tests
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2585549
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840258}