Fix MojoApplicationHost assertion when navigating from RenderProcessGone.

Termination of a renderer process results in a call to
WebContentsObserver::RenderProcessGone. Navigating from
RenderProcessGone using NavigationControllerImpl::Reload or LoadURL
results in an assertion due to MojoApplicationHost::Init() being called
again on an already initialized host. Reset |mojo_application_host_| in
RenderProcessHostImpl::ProcessDied before calling
ViewHostMsg_RenderProcessGone message handlers to avoid this problem.

BUG=448435

Review URL: https://codereview.chromium.org/848073002

Cr-Commit-Position: refs/heads/master@{#314435}

content/browser/renderer_host/render_process_host_impl.cc

@@ -2068,6 +2068,10 @@ void RenderProcessHostImpl::ProcessDied(bool already_dead,
 #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(
@@ -2077,8 +2081,6 @@ void RenderProcessHostImpl::ProcessDied(bool already_dead,
     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_)