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

ppapi/proxy/host_dispatcher.cc

 // Copyright (c) 2011 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.
 
 #include "ppapi/proxy/host_dispatcher.h"
 
 #include <map>
 
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
@@ skipping 116 matching lines @@
 }
 
 bool HostDispatcher::IsPlugin() const {
   return false;
 }
 
 bool HostDispatcher::Send(IPC::Message* msg) {
   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);
 }
 
 bool HostDispatcher::OnMessageReceived(const IPC::Message& msg) {
   TRACE_EVENT2("ppapi proxy", "HostDispatcher::OnMessageReceived",
                "Class", IPC_MESSAGE_ID_CLASS(msg.type()),
                "Line", IPC_MESSAGE_ID_LINE(msg.type()));
   // We only want to allow reentrancy when the most recent message from the
@@ skipping 53 matching lines @@
   dispatcher_ = static_cast<HostDispatcher*>(dispatcher);
   dispatcher_->ppb_proxy()->AddRefModule(dispatcher_->pp_module());
 }
 
 ScopedModuleReference::~ScopedModuleReference() {
   dispatcher_->ppb_proxy()->ReleaseModule(dispatcher_->pp_module());
 }
 
 }  // namespace proxy
 }  // namespace ppapi