compositing: Only make the compositor responsible for initiating reflow

for CSS transitions and animations, not `requestAnimationFrame()`
callbacks.

In the case of the latter, the script thread will kick off the reflow if
it's necessary, so there's no need for the compositor to do it.

Some pages, like nytimes.com, like to call `requestAnimationFrame()`
without actually mutating the DOM in the callback. We should avoid
reflowing in this case.

components/compositing/compositor.rs

@@ -1191,10 +1191,13 @@ impl<Window: WindowMethods> IOCompositor<Window> {
             }
         }
 
-        // We still need to tick layout unfortunately, see things like #12749.
-        let msg = ConstellationMsg::TickAnimation(pipeline_id, AnimationTickType::Layout);
-        if let Err(e) = self.constellation_chan.send(msg) {
-            warn!("Sending tick to constellation failed ({}).", e);
+        // We may need to tick animations in layout. (See #12749.)
+        let animations_running = self.pipeline_details(pipeline_id).animations_running;
+        if animations_running {
+            let msg = ConstellationMsg::TickAnimation(pipeline_id, AnimationTickType::Layout);
+            if let Err(e) = self.constellation_chan.send(msg) {
+                warn!("Sending tick to constellation failed ({}).", e);
+            }
         }
     }