[cast_bindings] Tests for MessagePort abstraction.

Test coverage for public MessagePort interface and its interactions with
MessagePort::Receiver. Includes conversion to and from the underlying
platform type of MessagePortCast or MessagePortFuchsia.

Bug: 1135377
Test: //components/cast/message_port:message_port_test added
Change-Id: I5e51aa5e08eb1e3e8957963b033497a94fe0bdac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515208
Commit-Queue: Shawn Quereshi <shawnq@google.com>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824126}