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>