Fabric: Remove redundant class forward declaration

Summary:
Remove redundant forward declaration and make default constructor explicit.

changelog: [internal]

Reviewed By: shergin

Differential Revision: D18032511

fbshipit-source-id: 18d3f9c869a69949e6965cd7e32e0f284723f2e3

ReactCommon/fabric/core/state/StateCoordinator.h

@@ -16,8 +16,6 @@
 namespace facebook {
 namespace react {
 
-class ShadowNode;
-
 /*
  * Coordinates a vision of the same state values between shadow nodes from
  * the same family.

ReactCommon/fabric/core/state/StateTarget.h

@@ -24,12 +24,12 @@ class StateTarget {
   /*
    * Creates an empty target.
    */
-  StateTarget();
+  explicit StateTarget();
 
   /*
    * Creates a target which points to a given `ShadowNode`.
    */
-  StateTarget(const ShadowNode &shadowNode);
+  explicit StateTarget(const ShadowNode &shadowNode);
 
   /*
    * Copyable and moveable.