Fabric: Using better::map instead of std::unordered_map in ShadowTreeRegistry

Summary:
Becasue better one is more efficient.

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: sammy-SC

Differential Revision: D18626666

fbshipit-source-id: 143dab1dd4c2e168368d2ca14bf2ffc8367d71ea

ReactCommon/fabric/core/primitives/ReactPrimitives.h

@@ -12,7 +12,6 @@
 #include <react/core/RawValue.h>
 #include <memory>
 #include <string>
-#include <unordered_map>
 
 namespace facebook {
 namespace react {

ReactCommon/fabric/mounting/ShadowTreeRegistry.h

@@ -7,6 +7,7 @@
 
 #pragma once
 
+#include <better/map.h>
 #include <better/mutex.h>
 
 #include <react/core/ReactPrimitives.h>
@@ -60,7 +61,7 @@ class ShadowTreeRegistry final {
 
  private:
   mutable better::shared_mutex mutex_;
-  mutable std::unordered_map<SurfaceId, std::unique_ptr<ShadowTree>>
+  mutable better::map<SurfaceId, std::unique_ptr<ShadowTree>>
       registry_; // Protected by `mutex_`.
 };