Adds associated interface support to IPC::Channel

Provides a way of registering associated interface bindings with
an IPC::Channel endpoint and acquiring proxies to remote interfaces
on the other side.

Support for this is exposed by IPC::Channel but only implemented
in IPC::ChannelMojo.

This is part a series of CLs to support Channel-associated interfaces.

BUG=612500

Committed: https://crrev.com/dc88e5075878f16cde23b910d2ce19aa862129e4
Review-Url: https://codereview.chromium.org/2137353002
Cr-Original-Commit-Position: refs/heads/master@{#405316}
Cr-Commit-Position: refs/heads/master@{#405364}

• ipc/BUILD.gn[diff]
• ipc/ipc.gyp[diff]
• ipc/ipc.mojom[diff]
• ipc/ipc_channel.h[diff]
• ipc/ipc_channel_common.cc[diff]
• ipc/ipc_channel_mojo.cc[diff]
• ipc/ipc_channel_mojo.h[diff]
• ipc/ipc_channel_mojo_unittest.cc[diff]
• ipc/ipc_message_pipe_reader.cc[diff]
• ipc/ipc_message_pipe_reader.h[diff]
• ipc/ipc_mojo_bootstrap.cc[diff]
• ipc/ipc_mojo_bootstrap.h[diff]
• ipc/ipc_test.mojom[Added - diff]