Implement autoplay gesture override experiment.

Source/core/layout/LayoutView.h

Index: Source/core/layout/LayoutView.h
diff --git a/Source/core/layout/LayoutView.h b/Source/core/layout/LayoutView.h
index ea2d26fb1293418ec90d06deeb704c0e50177288..a7f029a82a58f880fd02ffb3cd84efa2b3a10112 100644
--- a/Source/core/layout/LayoutView.h
+++ b/Source/core/layout/LayoutView.h
@@ -42,6 +42,7 @@ namespace blink {
 
 class DeprecatedPaintLayerCompositor;
 class LayoutQuote;
+class LayoutMedia;
 
 // The root of the layout tree, corresponding to the CSS initial containing block.
 // It's dimensions match that of the logical viewport (which may be different from
@@ -189,6 +190,15 @@ public:
     // It is very likely you do not want to call this method.
     void setShouldDoFullPaintInvalidationForViewAndAllDescendants();
 
+    // Some LayoutMedias want to know about their viewport visibility for
+    // crbug.com/487345,402044 .  This facility will be removed once those
+    // experiments complete.
+    void registerMediaForPositionChangeNotification(LayoutMedia*);
+    void unregisterMediaForPositionChangeNotification(LayoutMedia*);
+    // Notify all registered LayoutMedias that their position on-screen might
+    // have changed.
+    void sendMediaPositionChangeNotifications();
+
 private:
     void mapLocalToContainer(const LayoutBoxModelObject* paintInvalidationContainer, TransformState&, MapCoordinatesFlags = ApplyContainerFlip, bool* wasFixed = nullptr, const PaintInvalidationState* = nullptr) const override;
 
@@ -234,6 +244,8 @@ private:
     OwnPtrWillBePersistent<HitTestCache> m_hitTestCache;
 
     OwnPtrWillBePersistent<PendingSelection> m_pendingSelection;
+
+    Vector<LayoutMedia*> m_mediaForPositionNotification;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutView, isLayoutView());