Added a ResourceMessageFilter for handling resource messages on another thread.

ppapi/host/host_message_context.h

Index: ppapi/host/host_message_context.h
diff --git a/ppapi/host/host_message_context.h b/ppapi/host/host_message_context.h
index c9b3b3cf5a5b63c6fd41e55a30e7b64731666ea5..5a5486ec43738b6ec6758a0b0cbda510b0bba175 100644
--- a/ppapi/host/host_message_context.h
+++ b/ppapi/host/host_message_context.h
@@ -45,8 +45,9 @@ struct PPAPI_HOST_EXPORT HostMessageContext {
   // Returns a reply message context struct which includes the reply params.
   ReplyMessageContext MakeReplyMessageContext() const;
 
-  // The original call parameters passed to the resource message call.
-  const ppapi::proxy::ResourceMessageCallParams& params;
+  // The original call parameters passed to the resource message call. This
+  // cannot be a reference because this object may be passed to another thread.
+  ppapi::proxy::ResourceMessageCallParams params;
 
   // The reply message. If the params has the callback flag set, this message
   // will be sent in reply. It is initialized to the empty message. If the