Unify the calculation of main thread offset of sticky element

cc/trees/property_tree_builder.cc

Index: cc/trees/property_tree_builder.cc
diff --git a/cc/trees/property_tree_builder.cc b/cc/trees/property_tree_builder.cc
index a7efa0a71556ec6e68c77605d7e907fe504f32b3..a7a1af28292c3bd012bbedfe8618ba7cd3da6943 100644
--- a/cc/trees/property_tree_builder.cc
+++ b/cc/trees/property_tree_builder.cc
@@ -76,6 +76,14 @@ static LayerStickyPositionConstraint StickyPositionConstraint(
   return layer->test_properties()->sticky_position_constraint;
 }
 
+static gfx::Size StickyMainThreadOffset(Layer* layer) {
+  return layer->sticky_main_thread_offset();
+}
+
+static gfx::Size StickyMainThreadOffset(LayerImpl* layer) {
+  return layer->test_properties()->sticky_main_thread_offset;
+}
+
 static LayerImplList& Children(LayerImpl* layer) {
   return layer->test_properties()->children;
 }
@@ -540,14 +548,6 @@ bool AddTransformNodeIfNeeded(
             .AddNodeAffectedByOuterViewportBoundsDelta(node->id);
       }
     }
-    // TODO(smcgruer): Pass main thread sticky-shifting offsets of
-    // non-promoted ancestors, or promote all ancestor sticky elements.
-    // See http://crbug.com/702229
-    sticky_data->main_thread_offset =
-        layer->position().OffsetFromOrigin() -
-        sticky_data->constraints.parent_relative_sticky_box_offset
-            .OffsetFromOrigin();
-
     // Copy the ancestor nodes for later use. These layers are guaranteed to
     // have transform nodes at this point because they are our ancestors (so
     // have already been processed) and are sticky (so have transform nodes).
@@ -570,6 +570,9 @@ bool AddTransformNodeIfNeeded(
       DCHECK(sticky_data->nearest_node_shifting_containing_block !=
              TransformTree::kInvalidNodeId);
     }
+    node->sticky_main_thread_offset =
+        gfx::Vector2dF(StickyMainThreadOffset(layer).width(),
+                       StickyMainThreadOffset(layer).height());
   }
 
   node->needs_local_transform_update = true;