Implement autoplay gesture override experiment.

Source/core/layout/LayoutMedia.h

Index: Source/core/layout/LayoutMedia.h
diff --git a/Source/core/layout/LayoutMedia.h b/Source/core/layout/LayoutMedia.h
index 0b77b5dac25fc30e0c86971b83deff5d72310564..c11adf21c5b32260dab73c821af0102f66dabe9e 100644
--- a/Source/core/layout/LayoutMedia.h
+++ b/Source/core/layout/LayoutMedia.h
@@ -51,7 +51,21 @@ public:
 
     const char* name() const override { return "LayoutMedia"; }
 
+    // Temporary callback for crbug.com/587345,402044
+    void notifyPositionMayHaveChanged();
+
+    // Change whether we want or don't want to receive position change
+    // notifications.  This will cause us to start / stop receiving change
+    // notifications if possible.
+    // Temporary method for crbug.com/587345,402044
+    void setRequestPositionUpdates(bool);
+
 protected:
+    // Temporary overrides for crbug.com/587345,402044
+    void willBeDestroyed() override;
+    void insertedIntoTree() override;
+    void willBeRemovedFromTree() override;
+
     void layout() override;
 
     bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectMedia || LayoutImage::isOfType(type); }
@@ -70,7 +84,17 @@ private:
 
     bool backgroundShouldAlwaysBeClipped() const final { return false; }
 
+    // Register or unregister us for position change notifications, to match
+    // the argument.  Takes no action if not attached to a LayoutView or
+    // if we are already (un)registered correctly.
+    // Temporary helper for crbug.com/587345,402044
+    void updatePositionChangeRegistration(bool);
+
     LayoutObjectChildList m_children;
+
+    // Temporary state for crbug.com/587345,402044
+    bool m_registeredForPositionChangeNotifications;
+    bool m_wantPositionChangeNotifications;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutMedia, isMedia());