Fix a crash in the renderer process which occurs in the PPAPI host code due to the underlying plugin

ppapi/proxy/host_dispatcher.cc

Index: ppapi/proxy/host_dispatcher.cc
===================================================================
--- ppapi/proxy/host_dispatcher.cc	(revision 103325)
+++ ppapi/proxy/host_dispatcher.cc	(working copy)
@@ -134,11 +134,12 @@
   TRACE_EVENT2("ppapi proxy", "HostDispatcher::Send",
                "Class", IPC_MESSAGE_ID_CLASS(msg->type()),
                "Line", IPC_MESSAGE_ID_LINE(msg->type()));
-  // Normal sync messages are set to unblock, which would normally cause the
-  // plugin to be reentered to process them. We only want to do this when we
-  // know the plugin is in a state to accept reentrancy. Since the plugin side
-  // never clears this flag on messages it sends, we can't get deadlock, but we
-  // may still get reentrancy in the host as a result.
+  // Prevent the dispatcher from going away during the call. Scenarios
+  // where this could happen include a Send for a sync message which while
+  // waiting for the reply, dispatches an incoming ExecuteScript call which
+  // destroys the plugin module and in turn the dispatcher,
+  ScopedModuleReference ref(this);
+
   if (!allow_plugin_reentrancy_)
     msg->set_unblock(false);
   return Dispatcher::Send(msg);