WKWebViewConfiguration.allowsAirPlayForMediaPlayback is always true

[samman]

Created attachment 356108 [details]
The attachment is a screenshot of a test code that instantiates a WKWebView with a WKWebConfiguration whose `allowsAirPlayForMediaPlayback` is set as `false`, but fails when we assert this later.

The `allowsAirPlayForMediaPlayback` property of `WKWebView` is always true.
 
Attached is a screenshot of a test code that instantiates a WKWebView with a WKWebConfiguration whose `allowsAirPlayForMediaPlayback` is set as `false`. However, once a WKWebView gets initialized with the WKWebConfiguration, it's configuration's `allowsAirPlayForMediaPlayback` is always `true`

Steps to Reproduce:
```
        let config = WKWebViewConfiguration()
        config.allowsAirPlayForMediaPlayback = false
        
        let webView = WKWebView(frame: .zero, configuration: config)
        
        XCTAssertFalse(webView.configuration.allowsAirPlayForMediaPlayback)
```

Expected Results:
Assert statement should have passed.

Actual Results:
Assert statement fails.

[Radar WebKit Bug Importer]

<rdar://problem/46423666>