Fix the crash on exit with --single-process

In single process mode, RenderProcessObserverHelper got destroyed
before its host.

Change-Id: Ic2f36de3218f5ad6781610dbf677675b7e41a9eb
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>

src/core/user_script_controller_host.cpp

@@ -207,6 +207,8 @@ UserScriptControllerHost::UserScriptControllerHost()
 
 UserScriptControllerHost::~UserScriptControllerHost()
 {
+    Q_FOREACH (content::RenderProcessHost *renderer, m_observedProcesses)
+        renderer->RemoveObserver(m_renderProcessObserver.data());
 }
 
 } // namespace