Allow XMLHttpRequests from qrc to file

Add test for cross origin XMLHttpRequests from/to custom schemes. By default,
this is not allowed, but can be changed by adding an origin access whitelist
entry to blink::WebSecurityPolicy in the renderer.

Do this for the qrc scheme. As a result SecurityOrigin("qrc").CanRequest("file")
will return true, which makes DocumentThreadableLoader::Start disable CORS for
the request. Otherwise, CORS would be used, which only works with CORS enabled
schemes.

Fixes: QTBUG-70228
Change-Id: I2da60fddbbfb490c6d2f03329be286dbc28e1f12
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>