Fix links not opening on Google+ and Gmail

We missed a very important overload of WebContentsDelegate used by
window.open to navigate new pages. The logic is mostly taken from
chromium/content/shell/browser/shell.cc.

Change-Id: Ib359edb61c1ee6eab89e2ebdc2aad2d944fea712
Reviewed-by: Michael Bruning <michael.bruning@digia.com>