Sync support for new ios bridge

Reviewed By: javache

Differential Revision: D3897528

fbshipit-source-id: f30d2b1c490c1a9f7fc1f136858e6c7a146b9ca2

React/Base/RCTBridgeDelegate.h

@@ -63,6 +63,15 @@
  */
 - (NSArray<Class> *)whitelistedModulesForBridge:(RCTBridge *)bridge;
 
+/**
+ * When loading initial JavaScript, do so synchronously when the bridge is created iff
+ * this returns true.  Otherwise, the JS will be fetched on a network thread, and
+ * executed on the JS thread.  Currently used only by C++ bridge.
+ *
+ * @experimental
+ */
+- (BOOL)shouldBridgeLoadJavaScriptSynchronously:(RCTBridge *)bridge;
+
 /**
  * When initializing native modules that require main thread initialization, the bridge
  * will default to dispatch module creation blocks asynchrously. If we're blockingly