Cherry-pick 965cb88. rdar://115237220

    Unreviewed, fix the internal visionOS build
    rdar://115237220

    Add a missing `.get()` call when casting a `RetainPtr` to an Objective-C object of a different type;
    this now surfaces as a compiler warning in some versions of the visionOS SDK.

    * Source/WebKit/UIProcess/ios/fullscreen/WKFullScreenWindowControllerIOS.mm:
    (-[WKFullscreenWindowSceneDelegate methodSignatureForSelector:]):

    Canonical link: https://commits.webkit.org/267858@main

Source/WebKit/UIProcess/ios/fullscreen/WKFullScreenWindowControllerIOS.mm

@@ -607,7 +607,7 @@ - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
 {
     NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];
     if (!signature)
-        signature = [(NSObject *)_originalDelegate methodSignatureForSelector:aSelector];
+        signature = [(NSObject *)_originalDelegate.get() methodSignatureForSelector:aSelector];
     return signature;
 }