Keep NaCl plugins used in app background pages alive when active.

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 <string>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/files/file_path.h"
@@ skipping 707 matching lines @@
                     ppapi::HostResource /* video_decoder */,
                     int32_t /* picture buffer id */)
 IPC_MESSAGE_ROUTED2(PpapiMsg_PPPVideoDecoder_PictureReady,
                     ppapi::HostResource /* video_decoder */,
                     PP_Picture_Dev /* output picture */)
 IPC_MESSAGE_ROUTED2(PpapiMsg_PPPVideoDecoder_NotifyError,
                     ppapi::HostResource /* video_decoder */,
                     PP_VideoDecodeError_Dev /* error */)
 #endif  // !defined(OS_NACL) && !defined(NACL_WIN64)
 
+// Reports to the browser that a plugin has been active.
+IPC_MESSAGE_CONTROL0(PpapiHostMsg_Keepalive)
+
 // -----------------------------------------------------------------------------
 // These are from the plugin to the renderer.
 
 // Reply to PpapiMsg_CreateChannel. The handle will be NULL if the channel
 // could not be established. This could be because the IPC could not be created
 // for some weird reason, but more likely that the plugin failed to load or
 // initialize properly.
 IPC_MESSAGE_CONTROL1(PpapiHostMsg_ChannelCreated,
                      IPC::ChannelHandle /* handle */)
 
@@ skipping 1260 matching lines @@
 IPC_MESSAGE_CONTROL1(PpapiHostMsg_Talk_RequestPermission,
                      PP_TalkPermission /* permission */)
 IPC_MESSAGE_CONTROL0(PpapiPluginMsg_Talk_RequestPermissionReply)
 IPC_MESSAGE_CONTROL0(PpapiHostMsg_Talk_StartRemoting)
 IPC_MESSAGE_CONTROL0(PpapiPluginMsg_Talk_StartRemotingReply)
 IPC_MESSAGE_CONTROL0(PpapiHostMsg_Talk_StopRemoting)
 IPC_MESSAGE_CONTROL0(PpapiPluginMsg_Talk_StopRemotingReply)
 IPC_MESSAGE_CONTROL1(PpapiPluginMsg_Talk_NotifyEvent, PP_TalkEvent /* event */)
 
 #endif  // !defined(OS_NACL) && !defined(NACL_WIN64)