Add URL on ViewStructure.

The URL will be typically used by views rendering HTML pages (like WebView),
and used in 2 cases

- Root node (containing the URL of the HTML page)
- Child nodes that represent hyperlinks (contains the hyperlink URL).

Fixes: 20902429
Test: manual verification
Test: android.assist.cts.WebViewTest verifies it

Change-Id: Ic49472f1fca89047369b7187176f54efd277e204