fix crash on exit when using the inspector

Destroy the DevToolsHttpHandler early on, because ~DevToolsHttpHandler
assumes the browser's UI thread is still running.
This fixes a crash on exit in tst_inspectorserver.

Change-Id: I98476231b709d60c249e58e17b09de70aed6792b
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>

src/core/web_engine_context.cpp

@@ -153,6 +153,7 @@ void WebEngineContext::destroy()
     // Flush the UI message loop before quitting.
     while (delegate->DoWork()) { }
     GLContextHelper::destroy();
+    m_devtools.reset(0);
     m_runLoop->AfterRun();
 
     // Force to destroy RenderProcessHostImpl by destroying BrowserMainRunner.