Fabric: Implemented StubViewTree::getRootStubView()

Summary:
Trivial.
Before that we didn't have a way to get a root node from a Tree object.
It's useful during debugging (and some coming changes use that).

Reviewed By: mdvacca

Differential Revision: D16342529

fbshipit-source-id: 27c7516f3b3fccc8d8b25d4d2748006f7958ae41

ReactCommon/fabric/mounting/stubs/StubViewTree.cpp

@@ -11,9 +11,14 @@ namespace react {
 StubViewTree::StubViewTree(ShadowView const &shadowView) {
   auto view = std::make_shared<StubView>();
   view->update(shadowView);
+  rootTag = shadowView.tag;
   registry[shadowView.tag] = view;
 }
 
+StubView const &StubViewTree::getRootStubView() const {
+  return *registry.at(rootTag);
+}
+
 void StubViewTree::mutate(ShadowViewMutationList const &mutations) {
   for (auto const &mutation : mutations) {
     switch (mutation.type) {

ReactCommon/fabric/mounting/stubs/StubViewTree.h

@@ -21,6 +21,9 @@ class StubViewTree {
 
   void mutate(ShadowViewMutationList const &mutations);
 
+  StubView const &getRootStubView() const;
+
+  Tag rootTag;
   std::unordered_map<Tag, StubView::Shared> registry{};
 };