Move IPC::MessageFilter and router to a separate file

ppapi/proxy/host_dispatcher.h

Index: ppapi/proxy/host_dispatcher.h
diff --git a/ppapi/proxy/host_dispatcher.h b/ppapi/proxy/host_dispatcher.h
index d66fecfe673f9e150624190b4d5f09a84215ec3d..c1ba15feb1144ca0691665e2094d00ea3a562fc9 100644
--- a/ppapi/proxy/host_dispatcher.h
+++ b/ppapi/proxy/host_dispatcher.h
@@ -12,7 +12,7 @@
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
 #include "base/process/process.h"
-#include "ipc/ipc_channel_proxy.h"
+#include "ipc/message_filter.h"
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/proxy/dispatcher.h"
 
@@ -31,7 +31,7 @@ class PPAPI_PROXY_EXPORT HostDispatcher : public Dispatcher {
   //
   // Note that there can be nested sync messages, so the begin/end status
   // actually represents a stack of blocking messages.
-  class SyncMessageStatusReceiver : public IPC::ChannelProxy::MessageFilter {
+  class SyncMessageStatusReceiver : public IPC::MessageFilter {
    public:
     // Notification that a sync message is about to be sent out.
     virtual void BeginBlockOnSyncMessage() = 0;