[macOS] Make FlutterEngine support multiple views

shell/platform/darwin/macos/framework/Headers/FlutterEngine.h

@@ -79,10 +79,9 @@ FLUTTER_DARWIN_EXPORT
 - (BOOL)runWithEntrypoint:(nullable NSString*)entrypoint;
 
 /**
- * The default `FlutterViewController` of this engine, if any.
+ * The `FlutterViewController` of this engine, if any.
  *
- * The default view is the first view added to the engine, always has ID
- * kFlutterDefaultViewId, and is used by legacy APIs that assume a single view.
+ * This view is used by legacy APIs that assume a single view.
  *
  * Setting this field from nil to a non-nil view controller also updates
  * the view controller's engine and ID.

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

@@ -54,7 +54,7 @@
  * the given controller, if successfully attached, will always have the default
  * view ID kFlutterDefaultViewId.
  *
- *  The engine holds a weak reference to the attached view controller.
+ * The engine holds a weak reference to the attached view controller.
  *
  * If the given view controller is already attached to an engine, this call
  * throws an assertion.