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Todate, we've only passed a single, simple message to open a file from media-app
to media-app-guest. To better support our use-cases we need a bidirectional pipe,
and some abstractions around the dispatching of messages.

This CL introduces a MessagePipe class to encapsulate the communication channel
and manage dispatch of requests and responses between the privileged and
unprivileged JS contexts in chrome://media-app. It still requires explicit
definitions of what messages either side can send and receive but allows for
these messages to be defined and used with less friction.

Bug: 996088, 1040328
Change-Id: I6179a5d176d74f0f37fa1809363e87a0ed2976e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2049404
Commit-Queue: Zain Afzal <zafzal@google.com>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746214}