Move fragment navigation into Document object

Move the `check_and_scroll_fragment()` method into Document, make the
mothod set the fragment of url after navigation, and use the
`perform_a_scroll()` method to scroll rather than an individual
method. Also removes the broken `Window.fragment` fields.

components/url/lib.rs

@@ -98,6 +98,10 @@ impl ServoUrl {
         Arc::make_mut(&mut self.0).set_password(pass)
     }
 
+    pub fn set_fragment(&mut self, fragment: Option<&str>) {
+        Arc::make_mut(&mut self.0).set_fragment(fragment)
+    }
+
     pub fn username(&self) -> &str {
         self.0.username()
     }