Adapt and reland old position sticky implementation

public/platform/WebLayerPositionConstraint.h

Index: public/platform/WebLayerPositionConstraint.h
diff --git a/public/platform/WebLayerPositionConstraint.h b/public/platform/WebLayerPositionConstraint.h
index 973a06faf00aa57edddeaf13f5d894a1e6ea055a..4f24cb91fed799a825d87a9fbb4ef24f1ce0cf95 100644
--- a/public/platform/WebLayerPositionConstraint.h
+++ b/public/platform/WebLayerPositionConstraint.h
@@ -26,6 +26,8 @@
 #ifndef WebLayerPositionConstraint_h
 #define WebLayerPositionConstraint_h
 
+#include "public/platform/WebRect.h"
+
 namespace blink {
 
 struct WebLayerPositionConstraint {
@@ -50,6 +52,37 @@ struct WebLayerPositionConstraint {
     }
 };
 
+// TODO(flackr): Combine with the fixed position constraint above?

[enne (OOO), 2015/09/17 16:57:40]

Yeah, I think this should probably be merged with the other constraints.

+struct WebLayerStickyPositionConstraint {
+    bool isSticky         : 1;
+    bool isAnchoredLeft   : 1;
+    bool isAnchoredRight  : 1;
+    bool isAnchoredTop    : 1;
+    bool isAnchoredBottom : 1;
+
+    float leftOffset;
+    float rightOffset;
+    float topOffset;
+    float bottomOffset;
+
+    WebRect absoluteStickyBoxRect;

[enne (OOO), 2015/09/17 16:57:40]

Are these values redundant with WebLayer's bounds and the offsets above? Or can
the sticky part be some subrect of a layer?

[flackr, 2015/09/22 22:25:32]

I don't think so, but please correct me if I am mistaken about the layer's
bounds. The sticky position depends on the element's in flow position. Is it
possible to determine where the layer was before it was transformed /
positioned? (e.g. if it's anchored to the top it must not move above it's in
flow position)

+    WebRect absoluteContainingBlockRect;
+
+    // TODO(flackr): Add fancy ctor and static creation sugar.
+    WebLayerStickyPositionConstraint()
+        : isSticky(false)
+        , isAnchoredLeft(false)
+        , isAnchoredRight(false)
+        , isAnchoredTop(false)
+        , isAnchoredBottom(false)
+        , leftOffset(0.f)
+        , rightOffset(0.f)
+        , topOffset(0.f)
+        , bottomOffset(0.f)
+    {
+    }
+};
+
 } // namespace blink
 
 #endif // WebLayerPositionConstraint_h