Make mojo::embedder::ConnectToSlave() a bit more "correct".

Now it actually establishes a |Channel| and an initial message pipe.
(The implementation is in mojo::system::IPCSupport.) The old
IPCSupport::ConnectToSlave() was renamed ConnectToSlaveInternal() and
made private.

ConnectToMaster() needs to have the "same" thing done to it, at which
point I'll be able to test the two together.

(Also to do: Make the management of the channels created by
|ConnectToSlave()| more sane.)

R=yzshen@chromium.org

Review URL: https://codereview.chromium.org/1166303002.
diff --git a/shell/child_process_host.h b/shell/child_process_host.h
index 79fb292..67f243d 100644
--- a/shell/child_process_host.h
+++ b/shell/child_process_host.h
@@ -65,8 +65,8 @@
  private:
   struct LaunchData;
 
-  // Callback for |mojo::embedder::CreateChannel()|.
-  void DidCreateChannel(mojo::embedder::ChannelInfo* channel_info);
+  // Callback for |mojo::embedder::ConnectToSlave()|.
+  void DidConnectToSlave();
 
   // Note: This is probably executed on a different thread (namely, using the
   // blocking pool).