Fix MojoApplicationHost assertion when navigating from RenderProcessGone.

content/browser/renderer_host/render_process_host_impl.cc

 // 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.
 
 // Represents the browser side of the browser <--> renderer communication
 // channel. There will be one RenderProcessHost per renderer process.
 
 #include "content/browser/renderer_host/render_process_host_impl.h"
 
 #include <algorithm>
@@ skipping 2021 matching lines @@
   child_process_launcher_.reset();
   channel_.reset();
 
   within_process_died_observer_ = true;
   NotificationService::current()->Notify(
       NOTIFICATION_RENDERER_PROCESS_CLOSED,
       Source<RenderProcessHost>(this),
       Details<RendererClosedDetails>(&details));
   FOR_EACH_OBSERVER(RenderProcessHostObserver,
                     observers_,
                     RenderProcessExited(this, status, exit_code));

[Sam McNally, 2015/02/03 04:28:59]

Could RenderProcessExited or NOTIFICATION_RENDERER_PROCESS_CLOSED observers do
anything that requires mojo_application_host_ to be valid?

[Marshall, 2015/02/03 19:48:24]

A quick survey via codesearch didn't indicate any such uses to me, but I'm not
completely sure what "requires mojo_application_host_ to be valid" would look
like. Hopefully any such requirement will be caught by the CQ trybots while
testing this change.

   within_process_died_observer_ = false;
 
   gpu_message_filter_ = NULL;
   message_port_message_filter_ = NULL;
 #if defined(ENABLE_BROWSER_CDMS)
   browser_cdm_manager_ = NULL;
 #endif
   RemoveUserData(kSessionStorageHolderKey);
 
+  // RenderProcessGone handlers might navigate or perform other actions that
+  // require a connection. Ensure that there is one before calling them.
+  mojo_application_host_.reset(new MojoApplicationHost);
+
   IDMap<IPC::Listener>::iterator iter(&listeners_);
   while (!iter.IsAtEnd()) {
     iter.GetCurrentValue()->OnMessageReceived(
         ViewHostMsg_RenderProcessGone(iter.GetCurrentKey(),
                                       static_cast<int>(status),
                                       exit_code));
     iter.Advance();
   }
 
-  mojo_application_host_.reset(new MojoApplicationHost);
-
   // It's possible that one of the calls out to the observers might have caused
   // this object to be no longer needed.
   if (delayed_cleanup_needed_)
     Cleanup();
 
   // This object is not deleted at this point and might be reused later.
   // TODO(darin): clean this up
 }
 
 int RenderProcessHostImpl::GetActiveViewCount() {
@@ skipping 286 matching lines @@
 
 void RenderProcessHostImpl::DecrementWorkerRefCount() {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   DCHECK_GT(worker_ref_count_, 0);
   --worker_ref_count_;
   if (worker_ref_count_ == 0)
     Cleanup();
 }
 
 }  // namespace content