Allow MessageFilters to restrict listening to specific message classes

ipc/ipc_channel_proxy.h

Index: ipc/ipc_channel_proxy.h
diff --git a/ipc/ipc_channel_proxy.h b/ipc/ipc_channel_proxy.h
index 1f5ecf450df7f7c6d8a486a3d4efb76a95e1f81e..dca8c977302975e5fe9a0bf8c7dc188004d97121 100644
--- a/ipc/ipc_channel_proxy.h
+++ b/ipc/ipc_channel_proxy.h
@@ -88,6 +88,13 @@ class IPC_EXPORT ChannelProxy : public Sender, public base::NonThreadSafe {
     // the message be handled in the default way.
     virtual bool OnMessageReceived(const Message& message);
 
+    // Called to query the Message classes supported by the filter.  Return
+    // false to indicate that all message types should reach the filter, or true
+    // if the resulting contents of |supported_message_classes| may be used to
+    // selectively offer messages of a particular class to the filter.
+    virtual bool GetSupportedMessageClasses(
+        std::vector<uint32>* supported_message_classes) const;
+
    protected:
     virtual ~MessageFilter();
 
@@ -230,6 +237,11 @@ class IPC_EXPORT ChannelProxy : public Sender, public base::NonThreadSafe {
     std::string channel_id_;
     bool channel_connected_called_;
 
+    // Routes a given message to a proper subset of |filters_|, depending
+    // on which message classes a filter might support.
+    class MessageFilterRouter;
+    scoped_ptr<MessageFilterRouter> message_filter_router_;
+
     // Holds filters between the AddFilter call on the listerner thread and the
     // IPC thread when they're added to filters_.
     std::vector<scoped_refptr<MessageFilter> > pending_filters_;