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Changes the associated bindings implementation for ChannelMojo
such that remote interfaces can be acquired immediately upon
ChannelMojo construction rather than having to wait for connection
on the IO thread.

Simplifies the Channel bootstrapping process, removing a round-trip
Init message (and in fact the entire IPC::mojom::Boostrap interface)
since there's no need to actually exchange associated interface handles
over the pipe. Instead both sides can assume the other will use a fixed,
reserved endpoint ID for their IPC::mojom::Channel interface.

This also removes the restriction that associated interfaces must be
added to a Channel after Init. Instead the same constraints apply as
with AddFilter: an associated interface, like a filter, may be added
at any time as long as either Init hasn't been called OR the remote
process hasn't been launched.

The result of this CL is that any place it's safe to AddFilter,
it's also safe to AddAssociatedInterface; and any place it's safe to
Send, it's also safe to GetRemoteAssociatedInterface and begin using
any such remote interface immediately.

Remote interface requests as well as all messages to remote interfaces
retain FIFO with respect to any Send calls on the same thread. Local
interface request dispatch as well as all messages on locally bound
associated interfaces retain FIFO with respect to any OnMessageReceived
calls on the same thread.

BUG=612500,619202

Review-Url: https://codereview.chromium.org/2163633003
Cr-Commit-Position: refs/heads/master@{#406720}