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Also adds a simple service to allow layout tests to test all this.

This is part of a series of changes:
[1/5] Blink side changes in https://codereview.chromium.org/924983002/ that expose a needed v8 context
[2/5] https://codereview.chromium.org/921013002/ allow MessagePort to send messages as values
[3/5] https://codereview.chromium.org/944443003/ which causes the send_messages_as_values flag to be propagated
[4/5] This CL
[5/5] https://codereview.chromium.org/940423004/ adds layout tests to blink to test these new features

BUG=426458

Review URL: https://codereview.chromium.org/938403005

Cr-Commit-Position: refs/heads/master@{#318903}