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Adds a channel for sending messages to web content. Messages can include
MessagePorts to allow for bidirectional message exchange between the
FIDL client and web content.

Promotes Blink CloneableMessage and TransferableMessage struct traits
to "public" so that the messages can be serialized and deserialized
by code outside Blink.

Bug: 893236
Change-Id: If4e23a65e9d35e1ed2ce80de95e9f2212c55aecf
Reviewed-on: https://chromium-review.googlesource.com/c/1318839Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608515}