Add support for custom JSC across CS

Reviewed By: javache

Differential Revision: D3944510

fbshipit-source-id: 1c67c8a53a65149250a602f2ccd6b234a022897f

React/Executors/RCTJSCExecutor.h

@@ -53,6 +53,11 @@ RCT_EXTERN NSString *const RCTFBJSValueClassKey;
 
 - (instancetype)initWithUseCustomJSCLibrary:(BOOL)useCustomJSCLibrary;
 
+/**
+ * Marks whether the provider uses the custom implementation of JSC and not the system one.
+ */
+@property (nonatomic, readonly, assign) BOOL useCustomJSCLibrary;
+
 @end
 
 /**

React/Executors/RCTJSCExecutor.mm

@@ -944,7 +944,6 @@ RCT_EXPORT_METHOD(setContextName:(nonnull NSString *)name)
 @implementation RCTJSContextProvider
 {
   dispatch_semaphore_t _semaphore;
-  BOOL _useCustomJSCLibrary;
   NSThread *_javaScriptThread;
   JSContext *_context;
   RCTJSCWrapper *_jscWrapper;