rename main thread to platform thread

shell/platform/darwin/macos/framework/Source/FlutterEngine.mm

@@ -749,10 +749,10 @@ - (void)viewControllerViewDidLoad:(FlutterViewController*)viewController {
                       FlutterEngine* engine = weakSelf;
                       if (engine) {
                         // It is a bit unfortunate that embedder requires OnVSync call on
-                        // main thread just to immediately redispatch it to UI thread.
+                        // platform thread just to immediately redispatch it to UI thread.
                         // We are already on UI thread right now, but have to do the
                         // extra hop to main thread.
-                        [engine->_threadSynchronizer performOnMainThread:^{
+                        [engine->_threadSynchronizer performOnPlatformThread:^{
                           engine->_embedderAPI.OnVsync(_engine, baton, timeNanos, targetTimeNanos);
                         }];
                       }

shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizer.h

@@ -46,7 +46,7 @@
  * The block will be performed even if the platform thread is blocked waiting
  * for a commit.
  */
-- (void)performOnMainThread:(nonnull dispatch_block_t)block;
+- (void)performOnPlatformThread:(nonnull dispatch_block_t)block;
 
 /**
  * Requests the synchronizer to track another view.

shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizer.mm

@@ -164,7 +164,7 @@ - (void)performCommitForView:(int64_t)viewId
   event.Wait();
 }
 
-- (void)performOnMainThread:(nonnull dispatch_block_t)block {
+- (void)performOnPlatformThread:(nonnull dispatch_block_t)block {
   std::unique_lock<std::mutex> lock(_mutex);
   _scheduledBlocks.push_back(block);
   if (_beginResizeWaiting) {