WebChannel: don't mess with the navigator object

The navigator object is actually exposed later on from WebCore using
a different mechanism, so the V8 extension approach won't work with it.
Better take the chrome extensions approach and expose our own.
This should "un-break" the part of the web that was broken by
eee48292

.

Change-Id: I991be2e12bb9ebcf60ace02721497c3fcd923c7f
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>