IPC: Make ChannelReader inherit from SupportsAttachmentBrokering.

ipc/ipc_sync_channel.h

Index: ipc/ipc_sync_channel.h
diff --git a/ipc/ipc_sync_channel.h b/ipc/ipc_sync_channel.h
index 35934850ed7cbf77b2123612648bf86c65d17544..8b4b9a96e363e17187ef6f5e4efe07644628e68d 100644
--- a/ipc/ipc_sync_channel.h
+++ b/ipc/ipc_sync_channel.h
@@ -68,13 +68,17 @@ class IPC_EXPORT SyncChannel : public ChannelProxy {
   // Creates and initializes a sync channel. If create_pipe_now is specified,
   // the channel will be initialized synchronously.
   // The naming pattern follows IPC::Channel.
+  // TODO(erikchen): Remove default parameter for |broker|. It exists only to
+  // make the upcoming refactor decomposable into smaller CLs.
+  // http://crbug.com/493414.
   static scoped_ptr<SyncChannel> Create(
       const IPC::ChannelHandle& channel_handle,
       IPC::Channel::Mode mode,
       Listener* listener,
       const scoped_refptr<base::SingleThreadTaskRunner>& ipc_task_runner,
       bool create_pipe_now,
-      base::WaitableEvent* shutdown_event);
+      base::WaitableEvent* shutdown_event,
+      AttachmentBroker* broker = nullptr);
 
   static scoped_ptr<SyncChannel> Create(
       scoped_ptr<ChannelFactory> factory,