Ignore certain messages in plugin broker process if they are not sent by the

ppapi/proxy/ppapi_messages.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Multiply-included message header, no traditional include guard.
 
 #include <stdint.h>
 
 #include <map>
 #include <string>
@@ skipping 504 matching lines @@
 #endif  // !defined(OS_NACL) && !defined(NACL_WIN64)
 
 // These are from the browser to the plugin.
 // Loads the given plugin.
 IPC_MESSAGE_CONTROL2(PpapiMsg_LoadPlugin,
                      base::FilePath /* path */,
                      ppapi::PpapiPermissions /* permissions */)
 
 // Creates a channel to talk to a renderer. The plugin will respond with
 // PpapiHostMsg_ChannelCreated.
+// If |renderer_pid| is base::kNullProcessId, this is a channel used by the
+// browser itself.
 IPC_MESSAGE_CONTROL3(PpapiMsg_CreateChannel,
                      base::ProcessId /* renderer_pid */,
                      int /* renderer_child_id */,
                      bool /* incognito */)
 
 // Initializes the IPC dispatchers in the NaCl plugin.
 IPC_MESSAGE_CONTROL1(PpapiMsg_InitializeNaClDispatcher,
                      ppapi::PpapiNaClPluginArgs /* args */)
 
 // Instructs the plugin process to crash.
@@ skipping 1957 matching lines @@
                      std::vector<ppapi::HostResource> /* buffers */,
                      uint32_t /* buffer_size */)
 IPC_MESSAGE_CONTROL1(PpapiPluginMsg_VideoCapture_OnStatus,
                      uint32_t /* status */)
 IPC_MESSAGE_CONTROL1(PpapiPluginMsg_VideoCapture_OnError,
                      uint32_t /* error */)
 IPC_MESSAGE_CONTROL1(PpapiPluginMsg_VideoCapture_OnBufferReady,
                      uint32_t /* buffer */)
 
 #endif  // !defined(OS_NACL) && !defined(NACL_WIN64)