Fix Http User-agent string setting

Chromium has multiple places where it reads the user-agent, we were
only setting one, which appears to no longer be used for most cases.
This instead sets it by using the webcontent user-agent override.

Change-Id: I77d4d0a21424feda7a18140fb72baacdd6f816ca
Reviewed-by: Pierre Rossi <pierre.rossi@theqtcompany.com>