Adapt and reland old position sticky implementation

public/platform/WebLayer.h

Index: public/platform/WebLayer.h
diff --git a/public/platform/WebLayer.h b/public/platform/WebLayer.h
index 83089ca7248ee0fa599542965cbdc6f3bb3a497e..539f111aaf5f0417e909d81418e34a6ff2804b01 100644
--- a/public/platform/WebLayer.h
+++ b/public/platform/WebLayer.h
@@ -32,6 +32,7 @@
 #include "WebCompositorAnimation.h"
 #include "WebDoublePoint.h"
 #include "WebFloatPoint3D.h"
+#include "WebLayerPositionConstraint.h"
 #include "WebPoint.h"
 #include "WebRect.h"
 #include "WebScrollBlocksOn.h"
@@ -49,6 +50,7 @@ class WebLayerClient;
 class WebLayerScrollClient;
 struct WebFloatPoint;
 struct WebLayerPositionConstraint;
+struct WebLayerStickyPositionConstraint;
 
 class WebLayer {
 public:
@@ -220,6 +222,10 @@ public:
     virtual void setPositionConstraint(const WebLayerPositionConstraint&) = 0;
     virtual WebLayerPositionConstraint positionConstraint() const = 0;
 
+    // FIXME: Make pure once cc is updated.
+    virtual void setStickyPositionConstraint(const WebLayerStickyPositionConstraint&) { };
+    virtual WebLayerStickyPositionConstraint stickyPositionConstraint() const { return WebLayerStickyPositionConstraint(); }
+
     // The scroll client is notified when the scroll position of the WebLayer
     // changes. Only a single scroll client can be set for a WebLayer at a time.
     // The WebLayer does not take ownership of the scroll client, and it is the