introduce layout query timestamp

components/script_layout_interface/message.rs

@@ -24,6 +24,7 @@ use style::context::QuirksMode;
 use style::properties::PropertyId;
 use style::selector_parser::PseudoElement;
 use style::stylesheets::Stylesheet;
+use time;
 
 /// Asynchronous messages that script can send to layout.
 pub enum Msg {
@@ -171,6 +172,26 @@ impl ReflowGoal {
             },
         }
     }
+
+    /// Set the timestamp of query message.
+    pub fn set_timestamp(&mut self) {
+       match *self {
+           ReflowGoal::LayoutQuery(_, ref mut timestamp) => {
+               *timestamp = time::precise_time_ns();
+           },
+           _ => (),
+       }
+    }
+
+    /// Get the query timestamp.
+    pub fn timestamp(&self) -> Option<u64> {
+        match *self {
+            ReflowGoal::LayoutQuery(_, ref timestamp) => {
+                Some(*timestamp)
+            },
+            _ => None,
+        }
+    }
 }
 
 /// Information needed for a reflow.