servo: Merge #10225 - Implement non-visible pipeline and iframe visibility methods (from jmr0:visibility_api); r=jdm

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

Source-Repo: https://github.com/servo/servo
Source-Revision: d620ab71c41431c3fb040162f554faefb9abfbd7

servo/components/script_traits/lib.rs

@@ -163,6 +163,10 @@ pub enum ConstellationControlMsg {
     Freeze(PipelineId),
     /// Notifies script thread to resume all its timers
     Thaw(PipelineId),
+    /// Notifies script thread whether frame is visible
+    ChangeFrameVisibilityStatus(PipelineId, bool),
+    /// Notifies script thread that frame visibility change is complete
+    NotifyVisibilityChange(PipelineId, PipelineId, bool),
     /// Notifies script thread that a url should be loaded in this iframe.
     Navigate(PipelineId, SubpageId, LoadData),
     /// Requests the script thread forward a mozbrowser event to an iframe it owns
@@ -451,6 +455,8 @@ pub enum MozBrowserEvent {
     UsernameAndPasswordRequired,
     /// Sent when a link to a search engine is found.
     OpenSearch,
+    /// Sent when visibility state changes.
+    VisibilityChange(bool),
 }
 
 impl MozBrowserEvent {
@@ -472,7 +478,8 @@ impl MozBrowserEvent {
             MozBrowserEvent::ShowModalPrompt(_, _, _, _) => "mozbrowsershowmodalprompt",
             MozBrowserEvent::TitleChange(_) => "mozbrowsertitlechange",
             MozBrowserEvent::UsernameAndPasswordRequired => "mozbrowserusernameandpasswordrequired",
-            MozBrowserEvent::OpenSearch => "mozbrowseropensearch"
+            MozBrowserEvent::OpenSearch => "mozbrowseropensearch",
+            MozBrowserEvent::VisibilityChange(_) => "mozbrowservisibilitychange",
         }
     }
 }