Fabric: `LayoutableShadowNode:getLayoutMetrics` is not a virtual method anymore

Summary:
We don't use it as vitrual anymore (setLayoutMetrics is a non-virtual method already), so it does not need to be marker virtual.

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: mdvacca

Differential Revision: D21028572

fbshipit-source-id: 99f86fdd4cf2f5972034d9058d7b82bdc8680187

ReactCommon/fabric/core/layout/LayoutableShadowNode.h

@@ -89,7 +89,7 @@ class LayoutableShadowNode : public ShadowNode {
   /*
    * Returns layout metrics computed during previous layout pass.
    */
-  virtual LayoutMetrics getLayoutMetrics() const;
+  LayoutMetrics getLayoutMetrics() const;
 
   /*
    * Returns a transform object that represents transformations that will/should