Handle new window/tab modifiers also on normal navigations

AddNewContents is only called when window.open is called from JavaScript.

We also want the shift/ctrl modifiers to trigger normal links to open
in the requested disposition. In this case OpenURLFromTab will pass
the detected disposition and we must create a new WebContents
ourselves.

Use the same code path going through
WebContentsAdapterClient::adoptNewWindow except that we pass a null
WebContents pointer to the WebContentsAdapter constructor and let it
create its own when initialized.

Change-Id: I817b0e72aec12723bf92d9b7ad85c1cecbf5e408
Reviewed-by: Andras Becsi <andras.becsi@digia.com>