Clear HTTP cache before disabling it

It is a workaround to satisfy this online cache test:
http://refreshyourcache.com/en/cache-test/


This way Chromium provides "cache-control: no-cache" headers and indicates
the disabled state of our cache backend.

Task-number: QTBUG-63606
Change-Id: I0fdaf0b8dcaa98891b3d0380f4deeaa28a8aba4b
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>