[macOS] Fix FlutterViewController retain cycle (#42317)

Fixes flutter/flutter#127572

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: knopp

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

@@ -292,7 +292,7 @@ void OnKeyboardLayoutChanged(CFNotificationCenterRef center,
 
 @implementation FlutterViewWrapper {
   FlutterView* _flutterView;
-  FlutterViewController* _controller;
+  __weak FlutterViewController* _controller;
 }
 
 - (instancetype)initWithFlutterView:(FlutterView*)view

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

@@ -69,6 +69,7 @@ - (bool)testViewWillAppearCalledMultipleTimes;
 - (bool)testFlutterViewIsConfigured;
 - (bool)testLookupKeyAssets;
 - (bool)testLookupKeyAssetsWithPackage;
+- (bool)testViewControllerIsReleased;
 
 + (void)respondFalseForSendEvent:(const FlutterKeyEvent&)event
                         callback:(nullable FlutterKeyEventCallback)callback
@@ -251,6 +252,10 @@ id MockGestureEvent(NSEventType type, NSEventPhase phase, double magnification,
   ASSERT_TRUE([[FlutterViewControllerTestObjC alloc] testLookupKeyAssetsWithPackage]);
 }
 
+TEST(FlutterViewControllerTest, testViewControllerIsReleased) {
+  ASSERT_TRUE([[FlutterViewControllerTestObjC alloc] testViewControllerIsReleased]);
+}
+
 }  // namespace flutter::testing
 
 #pragma mark - FlutterViewControllerTestObjC
@@ -1016,4 +1021,24 @@ - (bool)testModifierKeysAreSynthesizedOnMouseMove {
   return true;
 }
 
+- (bool)testViewControllerIsReleased {
+  __weak FlutterViewController* weakController;
+  @autoreleasepool {
+    id engineMock = flutter::testing::CreateMockFlutterEngine(@"");
+
+    FlutterRenderer* renderer_ = [[FlutterRenderer alloc] initWithFlutterEngine:engineMock];
+    OCMStub([engineMock renderer]).andReturn(renderer_);
+
+    FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:engineMock
+                                                                                  nibName:@""
+                                                                                   bundle:nil];
+    [viewController loadView];
+    weakController = viewController;
+  }
+
+  EXPECT_EQ(weakController, nil);
+
+  return true;
+}
+
 @end