RCTSurface: Optional sync ShadowView/View registing

Summary:
See the comment in code.
If we on the main thread on registering time, we can/will break sequentiality of registering and mounting processes doing registration asynchronously.
We need this to make sync mouting eventually possible.

Reviewed By: fkgozali

Differential Revision: D7014176

fbshipit-source-id: 110ad5e5d86e3422eac15c3b1bdb29ae04acd7e6

React/Base/Surface/RCTSurface.mm

@@ -321,7 +321,9 @@ - (void)_registerRootView
 
   RCTUIManager *uiManager = batchedBridge.uiManager;
 
-  RCTExecuteOnUIManagerQueue(^{
+  // If we are on the main queue now, we have to proceed synchronously.
+  // Otherwise, we cannot perform synchronous waiting for some stages later.
+  (RCTIsMainQueue() ? RCTUnsafeExecuteOnUIManagerQueueSync : RCTExecuteOnUIManagerQueue)(^{
     [uiManager registerRootViewTag:self->_rootViewTag];
 
     RCTSurfaceRootShadowView *rootShadowView =