Unify the calculation of main thread offset of sticky element

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index d3100f3daa802322ca072e5be186709b65020bce..c20a6d03380a1f2bb389ccd43f3719f1f91cb556 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -597,18 +597,6 @@ void Layer::SetPosition(const gfx::PointF& position) {
           property_trees->transform_tree.UpdateNodeFromOwningLayerId(id())) {
     DCHECK_EQ(transform_tree_index(), transform_node->id);
     transform_node->update_post_local_transform(position, transform_origin());
-    if (transform_node->sticky_position_constraint_id >= 0) {
-      StickyPositionNodeData* sticky_data =
-          property_trees->transform_tree.StickyPositionData(
-              transform_tree_index());
-      // 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 =
-          position.OffsetFromOrigin() -
-          sticky_data->constraints.parent_relative_sticky_box_offset
-              .OffsetFromOrigin();
-    }
     transform_node->needs_local_transform_update = true;
     transform_node->transform_changed = true;
     layer_tree_host_->property_trees()->transform_tree.set_needs_update(true);
@@ -1108,6 +1096,15 @@ void Layer::SetStickyPositionConstraint(
   SetNeedsCommit();
 }
 
+void Layer::SetStickyMainThreadOffset(const gfx::Size& offset) {
+  DCHECK(IsPropertyChangeAllowed());
+  if (inputs_.sticky_main_thread_offset == offset)
+    return;
+  inputs_.sticky_main_thread_offset = offset;
+  SetPropertyTreesNeedRebuild();

[smcgruer, 2017/05/26 17:14:12]

Do we definitely need to rebuild property trees here? Could we not propagate to
the existing TransformNode in this method instead?

[yigu, 2017/05/26 18:04:21]

We don't need to rebuild the tree. Done.

+  SetNeedsCommit();
+}
+
 static void RunCopyCallbackOnMainThread(
     std::unique_ptr<CopyOutputRequest> request,
     std::unique_ptr<CopyOutputResult> result) {