Unify the calculation of main thread offset of sticky element

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index c88721b0fa6e1e82ad3728900e1d3f93b7cc566b..166ecfbfa26cab82cc89a11f5017529da74dc6d8 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -174,6 +174,11 @@ class CC_EXPORT Layer : public base::RefCounted<Layer> {
     return inputs_.sticky_position_constraint;
   }
 
+  void SetStickyMainThreadOffset(const gfx::Size& offset);
+  const gfx::Size sticky_main_thread_offset() const {
+    return inputs_.sticky_main_thread_offset;
+  }
+
   void SetTransform(const gfx::Transform& transform);
   const gfx::Transform& transform() const { return inputs_.transform; }
 
@@ -577,6 +582,7 @@ class CC_EXPORT Layer : public base::RefCounted<Layer> {
     LayerPositionConstraint position_constraint;
 
     LayerStickyPositionConstraint sticky_position_constraint;
+    gfx::Size sticky_main_thread_offset;
 
     ElementId element_id;