[ReactNative] Use JSValueIsUndefined instead of comparing with JSValu…

…eMakeUndefined

Summary:
@public

Use JSValueIsUndefined instead of caching an JSValueMakeUndefined to compare with
as suggested in facebook#1432 (comment)

Test Plan: Run the RCTContextExecutor tests

React/Executors/RCTContextExecutor.m

@@ -66,7 +66,6 @@ @implementation RCTContextExecutor
 {
   RCTJavaScriptContext *_context;
   NSThread *_javaScriptThread;
-  JSValueRef _undefined;
 }
 
 /**
@@ -238,9 +237,6 @@ - (instancetype)initWithJavaScriptThread:(NSThread *)javaScriptThread
         JSContextGroupRelease(group);
       }
 
-      // Constant value used for comparison
-      _undefined = JSValueMakeUndefined(ctx);
-
       strongSelf->_context = [[RCTJavaScriptContext alloc] initWithJSContext:ctx];
       [strongSelf _addNativeHook:RCTNativeLoggingHook withName:"nativeLoggingHook"];
       [strongSelf _addNativeHook:RCTNoop withName:"noop"];
@@ -312,7 +308,7 @@ - (void)executeJSCall:(NSString *)name
     JSValueRef requireJSRef = JSObjectGetProperty(contextJSRef, globalObjectJSRef, requireNameJSStringRef, &errorJSRef);
     JSStringRelease(requireNameJSStringRef);
 
-    if (requireJSRef != NULL && requireJSRef != _undefined && errorJSRef == NULL) {
+    if (requireJSRef != NULL && !JSValueIsUndefined(contextJSRef, requireJSRef) && errorJSRef == NULL) {
 
       // get module
       JSStringRef moduleNameJSStringRef = JSStringCreateWithCFString((__bridge CFStringRef)name);