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In preparation for adding mojo Blob handles to the serialized message
port messages, this changes the logic in content::MessagePort to
serialize its messages using a mojom bindings generated struct. For now
the struct still just contains the blink encoded message and a list of
message port handles, but in the future blobs and other extra data can
be added much easier.

Bug: 351753, 740744
Change-Id: I411dd4398907eb7bbf21e630f5c82fb49798c440
Reviewed-on: https://chromium-review.googlesource.com/590082
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Darin Fisher <darin@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490628}