Do not assume that the webview only contains RWHV delegates

This fixes an assertion for QML code like:

WebEngineView {
    ...
    MouseArea {
       ...
    }
}

Change-Id: I7a9bfb801c81ea53c1cc830b44fe224a0b78db2d
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>