PPAPI/NaCl: Move plugin_main.cc's contents into irt_ppapi.cc

ppapi/nacl_irt/plugin_startup.h

Index: ppapi/nacl_irt/plugin_startup.h
diff --git a/ppapi/nacl_irt/plugin_startup.h b/ppapi/nacl_irt/plugin_startup.h
index ea14836e60eb43a33c0481011c70b45e2bbaf2cd..2684a78316bc821e670bb6d2f9d926b17b1efb1f 100644
--- a/ppapi/nacl_irt/plugin_startup.h
+++ b/ppapi/nacl_irt/plugin_startup.h
@@ -17,8 +17,8 @@ namespace ppapi {
 class ManifestService;
 
 // Sets the IPC channels for the browser and the renderer by the given FD
-// numbers. This will be used for non-SFI mode. Must be called before
-// PpapiPluginMain is called.
+// numbers. This will be used for non-SFI mode. Must be called before the
+// ppapi_start() IRT interface is called.
 PPAPI_PROXY_EXPORT void SetIPCFileDescriptors(
     int browser_ipc_fd, int renderer_ipc_fd, int manifest_service_fd);