Fix toPlainText related crashes in QtRenderViewObserver

Use the WebFrame's more reliable contentAsMarkup() and
contentAsText() methods instead of going down to the Document
element.

Change-Id: I332e24375e074124099b59e8a784a0a038c6f70b
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>