Don't create the JSCRuntime until createJSExecutor() is called.

Summary:
This avoids an intermittent reentrancy bug in JSC on iOS 11
(https://bugs.webkit.org/show_bug.cgi?id=186827).  It also makes the
code more consistent with android.

Reviewed By: amnn

Differential Revision: D13313265

fbshipit-source-id: f42476b2f660e127ecfc9c72584554817eea1010

React/CxxBridge/RCTCxxBridge.mm

@@ -56,7 +56,6 @@
 
 typedef void (^RCTPendingCall)();
 
-using namespace facebook::jsc;
 using namespace facebook::jsi;
 using namespace facebook::react;
 
@@ -92,6 +91,24 @@ typedef NS_ENUM(NSUInteger, RCTBridgeFields) {
   std::shared_ptr<JSExecutorFactory> factory_;
 };
 
+class JSCExecutorFactory : public JSExecutorFactory {
+public:
+  std::unique_ptr<JSExecutor> createJSExecutor(
+    std::shared_ptr<ExecutorDelegate> delegate,
+    std::shared_ptr<MessageQueueThread> jsQueue) override {
+    return folly::make_unique<JSIExecutor>(
+      facebook::jsc::makeJSCRuntime(),
+      delegate,
+      [](const std::string &message, unsigned int logLevel) {
+        _RCTLogJavaScriptInternal(
+          static_cast<RCTLogLevel>(logLevel),
+          [NSString stringWithUTF8String:message.c_str()]);
+      },
+      JSIExecutor::defaultTimeoutInvoker,
+      nullptr);
+  }
+};
+
 }
 
 static bool isRAMBundle(NSData *script) {
@@ -320,13 +337,7 @@ - (void)start
       executorFactory = [cxxDelegate jsExecutorFactoryForBridge:self];
     }
     if (!executorFactory) {
-      executorFactory = std::make_shared<JSIExecutorFactory>(
-          makeJSCRuntime(),
-          [](const std::string &message, unsigned int logLevel) {
-              _RCTLogJavaScriptInternal(
-                  static_cast<RCTLogLevel>(logLevel),
-                  [NSString stringWithUTF8String:message.c_str()]);
-          }, nullptr);
+      executorFactory = std::make_shared<JSCExecutorFactory>();
     }
   } else {
     id<RCTJavaScriptExecutor> objcExecutor = [self moduleForClass:self.executorClass];