Mojo: Refactor PlatformChannel stuff.

Mojo: Refactor PlatformChannel stuff.

- Remove PlatformServerChannel/PlatformClientChannel.
- Add PlatformChannelPair (move stuff formerly in PlatformServerChannel
  into this).

It became apparent that my previous plan to make this work on Windows
wasn't work nicely. On the one hand, on Vista+, we can basically make
things work like POSIX (created the channels in the parent and connect
them, and send a channel to a child). On the other, on XP, to be secure
you need to do more work (the channels aren't connected or authenticated
initially), so you'd need much more machinery (to wait for connection,
to authenticate, etc.).

So I'll go for a different mechanism to make things work on XP. The
assumption from the Mojo embedder API will be that it's given a channel
handle that's already been connected, authenticated, etc. (which will be
taken care of by other means). This will add flexibility in other ways
as well (e.g., make Mojo IPC more happily coexist with Chrome IPC -- you
should be able to pass a handle over Chrome IPC to set up Mojo IPC).

Still to do: Move PlatformChannelPair into its own files.

R=darin@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=244223

[darin (slow to review), 2014-01-10 19:53:35]

LGTM

[viettrungluu, 2014-01-10 20:03:22]

Committed patchset #1 manually as r244223 (presubmit successful).