[iOS] Adopt ThreadSafeWeakPtr in WebMediaSessionHelper

https://bugs.webkit.org/show_bug.cgi?id=260205
rdar://113910990

Reviewed by Jean-Yves Avenard.

Adopted ThreadSafeWeakPtr for WebMediaSessionHelper's _callback ivar. Ensured
WebMediaSessionHelper's _monitoringAirPlayRoutes and _startMonitoringAirPlayRoutesPending ivars are
accessed exclusively on the main thread. Fixed some naming and formatting issues, notably renaming
MediaSessionHelperiOS to MediaSessionHelperIOS.

* Source/WebCore/platform/audio/ios/MediaSessionHelperIOS.mm:
(MediaSessionHelper::resetSharedHelper):
(MediaSessionHelperIOS::MediaSessionHelperIOS):
(MediaSessionHelperIOS::providePresentingApplicationPID):
(MediaSessionHelperIOS::startMonitoringWirelessRoutesInternal):
(MediaSessionHelperIOS::stopMonitoringWirelessRoutesInternal):
(MediaSessionHelperIOS::mediaServerConnectionDied):
(MediaSessionHelperIOS::setIsPlayingToAutomotiveHeadUnit):
(MediaSessionHelperIOS::activeAudioRouteDidChange):
(MediaSessionHelperIOS::activeVideoRouteDidChange):
(MediaSessionHelperIOS::externalOutputDeviceAvailableDidChange):
(-[WebMediaSessionHelper initWithCallback:]):
(-[WebMediaSessionHelper hasWirelessTargetsAvailable]):
(-[WebMediaSessionHelper startMonitoringAirPlayRoutes]):
(-[WebMediaSessionHelper applicationWillEnterForeground:]):
(-[WebMediaSessionHelper applicationDidBecomeActive:]):
(-[WebMediaSessionHelper applicationWillResignActive:]):
(-[WebMediaSessionHelper wirelessRoutesAvailableDidChange:]):
(-[WebMediaSessionHelper applicationDidEnterBackground:]):
(-[WebMediaSessionHelper mediaServerConnectionDied:]):
(-[WebMediaSessionHelper carPlayIsConnectedDidChange:]):
(-[WebMediaSessionHelper activeOutputDeviceDidChange:]):
(MediaSessionHelperiOS::MediaSessionHelperiOS): Deleted.
(MediaSessionHelperiOS::~MediaSessionHelperiOS): Deleted.
(MediaSessionHelperiOS::providePresentingApplicationPID): Deleted.
(MediaSessionHelperiOS::startMonitoringWirelessRoutesInternal): Deleted.
(MediaSessionHelperiOS::stopMonitoringWirelessRoutesInternal): Deleted.
(MediaSessionHelperiOS::mediaServerConnectionDied): Deleted.
(MediaSessionHelperiOS::updateCarPlayIsConnected): Deleted.
(MediaSessionHelperiOS::setIsPlayingToAutomotiveHeadUnit): Deleted.
(MediaSessionHelperiOS::activeAudioRouteDidChange): Deleted.
(MediaSessionHelperiOS::activeVideoRouteDidChange): Deleted.
(MediaSessionHelperiOS::externalOutputDeviceAvailableDidChange): Deleted.
(-[WebMediaSessionHelper clearCallback]): Deleted.

Canonical link: https://commits.webkit.org/266985@main