Do not switch queues if not necessary

Summary:
Changelog: [Internal]

Switching queue here is not necessary if we are already on the main queue.

 This is important for Fabric SSTs, otherwise images are missing.

Reviewed By: shergin

Differential Revision: D19907908

fbshipit-source-id: 52e82484afc8e2f591d0c5cc126952990d992e96

Author: sammy-SC

React/Fabric/RCTImageResponseObserverProxy.mm

@@ -7,6 +7,7 @@
 
 #import "RCTImageResponseObserverProxy.h"
 
+#import <React/RCTUtils.h>
 #import <react/imagemanager/ImageResponse.h>
 #import <react/imagemanager/ImageResponseObserver.h>
 #import <react/utils/ManagedObjectWrapper.h>
@@ -24,7 +25,7 @@
   UIImage *image = (UIImage *)unwrapManagedObject(imageResponse.getImage());
   id<RCTImageResponseDelegate> delegate = delegate_;
   auto this_ = this;
-  dispatch_async(dispatch_get_main_queue(), ^{
+  RCTExecuteOnMainQueue(^{
     [delegate didReceiveImage:image fromObserver:this_];
   });
 }
@@ -33,7 +34,7 @@
 {
   auto this_ = this;
   id<RCTImageResponseDelegate> delegate = delegate_;
-  dispatch_async(dispatch_get_main_queue(), ^{
+  RCTExecuteOnMainQueue(^{
     [delegate didReceiveProgress:progress fromObserver:this_];
   });
 }
@@ -42,7 +43,7 @@
 {
   auto this_ = this;
   id<RCTImageResponseDelegate> delegate = delegate_;
-  dispatch_async(dispatch_get_main_queue(), ^{
+  RCTExecuteOnMainQueue(^{
     [delegate didReceiveFailureFromObserver:this_];
   });
 }