Fix WebContentsAdapater::requestedUrl() function

The requestedUrl function didn't return empty URL even if
the empty URL was really requested. It was assumed if
GetOriginalRequestURL returned empty string that means the requested url was
not set in the navigation entry.

This fix handles that case when empty url is set in the navigation
entry as requested url. If the navigation entry is in pending state that means the
request url has not been set yet thus the actual URL should be returned.

Change-Id: Ic2eff5c487686f7c0e349a7a34a86b80551a002f
Reviewed-by: Andras Becsi <andras.becsi@digia.com>