fix: check for legacy Fairplay support when switching out of handleEncryptedEvent

[Essk]

If we attempt to play content with both legacy Fairplay and Widevine DRM configured in Chrome, something like this:

{
    "com.apple.fps.1_0": {
    ......
    },

    "com.widevine.alpha": {
        "url": "xxxxxxx.xxxxx.xxxx",
    }
},

is passed to handleEncryptedEvent, where this check detects the presence of the legacy Fairplay configuration, exits out and, as we're in Chrome, playback can't start as Fairplay isn't supported.

This change adds a simple additional check for WebKitMediaKeys for confirmation that the platform supports legacy Fairplay before exiting out of handleEncryptedEvent.

[adrums86]