Fixed crash when chrome.runtime.reload() is called with app windows open

apps/app_window_contents.cc

Index: apps/app_window_contents.cc
diff --git a/apps/app_window_contents.cc b/apps/app_window_contents.cc
index d163c2ce84ace7bce1cdb270d9412ce4e87b2d98..7b88e67e6f6092e9d2b338d4f6be393fd1fa8ec5 100644
--- a/apps/app_window_contents.cc
+++ b/apps/app_window_contents.cc
@@ -96,7 +96,7 @@ void AppWindowContents::NativeWindowChanged(
 
   content::RenderViewHost* rvh = web_contents_->GetRenderViewHost();
   rvh->Send(new ExtensionMsg_MessageInvoke(rvh->GetRoutingID(),
-                                           host_->extension()->id(),
+                                           host_->extension_id(),

[Matt Perry, 2013/09/23 21:42:31]

Rather than patching this here, could you change ExtensionHost to hold a
scoped_refptr to the Extension object? That will prevent a use-after-free in
case we access the extension object elsewhere. (Though I think the deeper bug is
that the ExtensionHost isn't notified when the Extension object is freed.
Extension should outlive ExtensionHost.)

[tmdiep, 2013/09/24 00:33:47]

The host_ pointer is an apps::ShellWindow. The ShellWindow does receive the
extension unloaded notification. The patch submitted for this related issue sets
the extension pointer to null to prevent use after free, but it has been
reverted:
https://code.google.com/p/chromium/issues/detail?id=174250

After reloading the app, AppWindowContents::NativeWindowChanged is called as a
result of:
1. Widget::Close()
2. Widget::OnNativeWidgetVisibilityChanged(visible: false)
3. Widget::OnNativeWidgetActivationChanged(active: false)

The third call is the cause of the crash. I think we can check whether the
extension is null and not forward the message to the RenderViewHost.

                                            "app.window",
                                            "updateAppWindowProperties",
                                            args,