Adds Channel-associated interface support on ChannelProxy's thread

This exposes a way for ChannelProxy users to associate interface
factories with a ChannelProxy, allowing requests to be bound either
on the proxy thread or directly on the IPC thread.

BUG=612500

Review-Url: https://codereview.chromium.org/2147493006
Cr-Commit-Position: refs/heads/master@{#405504}

• ipc/ipc_channel.h[diff]
• ipc/ipc_channel_mojo.cc[diff]
• ipc/ipc_channel_mojo.h[diff]
• ipc/ipc_channel_mojo_unittest.cc[diff]
• ipc/ipc_channel_proxy.cc[diff]
• ipc/ipc_channel_proxy.h[diff]
• ipc/ipc_mojo_bootstrap.cc[diff]
• ipc/ipc_mojo_bootstrap.h[diff]