Auto merge of #10225 - jmr0:visibility_api, r=jdm

Implement non-visible pipeline and iframe visibility methods

This addresses #9566 and a good part of #9751, specifically:

* Pipeline has a notion of visibility
* IFrame setVisible/getVisible interface with IFrame's pipeline visibility
* IFrame mozbrowservisibilitychange responds to changes in visibility
* Pipeline visibility is used to limit animations (requestAnimationFrame does not tick animations when hidden) and to increase timer intervals (currently set to a minimum of 1 second while hidden)

Absent for now are any changes to the Document API and general implementation of the Page Visibility API, since the more interesting parts require knowledge of whether the user agent is minimized, OS screen locked, etc.

cc @paulrouget @jdm

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components/script/dom/window.rs

@@ -1509,6 +1509,14 @@ impl Window {
         self.timers.suspend();
     }
 
+    pub fn slow_down_timers(&self) {
+        self.timers.slow_down();
+    }
+
+    pub fn speed_up_timers(&self) {
+        self.timers.speed_up();
+    }
+
     pub fn need_emit_timeline_marker(&self, timeline_type: TimelineMarkerType) -> bool {
         let markers = self.devtools_markers.borrow();
         markers.contains(&timeline_type)