Changes ThreadSafeInterfacePtr such that it is no longer possible
to create one which is unbound. Instead, it is now possible to
create one which will imminently be bound on a specific TaskRunner
but which is usable immediately.

Also introduces ThreadSafeForwarder as a reduced encapsulation of
thread-safe serialization and forwarding logic. This is used
to implement ThreadSafeInterfacePtrBase, as well as to support
immediate associated request forwarding on IPC::ChannelProxy in
a thread-safe manner, where the underlying AssociatedInterfacePtr
must remain owned by the channel's IPC::MessagePipeReader.

Finally, in order to facilitate IPC::Channel exposing ipc.mojom
types through its public interface, the //ipc:mojom target has
been folded into //ipc's component exports. Any prior dependents
on //ipc:mojom have been updated accordingly.

BUG=682334
R=yzshen@chromium.org
TEST=ipc_tests, mojo_public_bindings_unittests

Review-Url: https://codereview.chromium.org/2668153003
Cr-Commit-Position: refs/heads/master@{#449239}