ChannelMojo: Remove intermediate Message struct

This passes the data and handles directly in the Receive
message rather than using an intermediate Message struct.
One less alloc per Send, which seems worthwhile even though
the performance delta is marginal.

R=sammc@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1809323002

Cr-Commit-Position: refs/heads/master@{#382037}
3 files changed