Fix duplicate document role

The blink root element already has document as role. By setting it in
the webview we'd get two documents in the hierarchy.

Change-Id: Ie14e57b4e2050dbd9495c4fa94de4e2faebc02c1
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Implement functions of BrowserAccessibilityDelegate

This is in line with how other platforms do it.

Change-Id: Ia258511d3fa35387a69c81c9c02c181fc2995e6c
Reviewed-by: Andras Becsi <andras.becsi@digia.com>