Make backgroundColor a property on FVC

Author: a-wallen

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

@@ -91,7 +91,7 @@ FLUTTER_DARWIN_EXPORT
  *     // If the `FlutterView`'s color is not set via `FlutterViewController.setBackgroundColor`
  *     // it's default will be black.
  *     self.backgroundColor = NSColor.clear
- *     flutterViewController.setBackgroundColor(NSColor.white)
+ *     flutterViewController.backgroundColor = NSColor.clear
  *
  *     let windowFrame = self.frame
  *     self.contentViewController = flutterViewController
@@ -104,6 +104,6 @@ FLUTTER_DARWIN_EXPORT
  * }
  * ```
  */
-- (void)setBackgroundColor:(nonnull NSColor*)color;
+@property(readwrite, nonatomic, nullable) NSColor* backgroundColor;
 
 @end

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

@@ -145,7 +145,7 @@ @interface FlutterEngine (Test)
   [viewController loadView];
   viewController.flutterView.frame = CGRectMake(0, 0, 800, 600);
   [engine setViewController:viewController];
-  [viewController setBackgroundColor:[NSColor whiteColor]];
+  viewController.flutterView.backgroundColor = [NSColor whiteColor];
 
   // Latch to ensure the entire layer tree has been generated and presented.
   fml::AutoResetWaitableEvent latch;

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

@@ -422,7 +422,8 @@ - (void)setMouseTrackingMode:(FlutterMouseTrackingMode)mode {
 }
 
 - (void)setBackgroundColor:(NSColor*)color {
-  [(FlutterViewWrapper*)[self view] setBackgroundColor:color];
+  _backgroundColor = color;
+  [(FlutterViewWrapper*)[self view] setBackgroundColor:_backgroundColor];
 }
 
 - (void)onPreEngineRestart {