[ios] Fix memory leak in ChildClippingView (#50389)

This fixes the memory leak in `ChildClippingView`.

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

Author: 0xZOne

AUTHORS

@@ -24,4 +24,6 @@ TheOneWithTheBraid <[email protected]>
 Twin Sun, LLC <[email protected]>
 Qixing Cao <[email protected]>
 LinXunFeng <[email protected]>
-Amir Panahandeh <[email protected]>
+Amir Panahandeh <[email protected]>
+Rulong Chen（陈汝龙）<[email protected]>
+

shell/platform/darwin/ios/framework/Source/FlutterPlatformViewsTest.mm

@@ -246,6 +246,17 @@ - (void)testChildClippingViewHitTests {
   XCTAssertTrue([childClippingView pointInside:CGPointMake(199, 199) withEvent:nil]);
 }
 
+- (void)testReleasesBackdropFilterSubviewsOnChildClippingViewDealloc {
+  __weak NSMutableArray<UIVisualEffectView*>* weakBackdropFilterSubviews = nil;
+  @autoreleasepool {
+    ChildClippingView* clipping_view = [[ChildClippingView alloc] initWithFrame:CGRectZero];
+    weakBackdropFilterSubviews = clipping_view.backdropFilterSubviews;
+    XCTAssertNotNil(weakBackdropFilterSubviews);
+    clipping_view = nil;
+  }
+  XCTAssertNil(weakBackdropFilterSubviews);
+}
+
 - (void)testApplyBackdropFilter {
   flutter::FlutterPlatformViewsTestMockPlatformViewDelegate mock_delegate;
   auto thread_task_runner = CreateNewThread("FlutterPlatformViewsTest");

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews_Internal.h

@@ -120,6 +120,8 @@
 // filters.
 - (void)applyBlurBackdropFilters:(NSArray<PlatformViewFilter*>*)filters;
 
+// For testing only.
+- (NSMutableArray*)backdropFilterSubviews;
 @end
 
 namespace flutter {

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews_Internal.mm

@@ -236,7 +236,7 @@ - (void)dealloc {
 
 - (NSMutableArray*)backdropFilterSubviews {
   if (!_backdropFilterSubviews) {
-    _backdropFilterSubviews = [[[NSMutableArray alloc] init] retain];
+    _backdropFilterSubviews = [[NSMutableArray alloc] init];
   }
   return _backdropFilterSubviews;
 }