widthMediaFeatureEval ends up with null FrameView during iframe unload.

widthMediaFeatureEval ends up with null FrameView during iframe unload.

[Title] widthMediaFeatureEval ends up with null FrameView during iframe unload.
[Issue#] N_SE-51174
[Problem] Crash while loading mashable.com
[Cause] While frames are being unloaded, the Frame object does not necessarily have valid
        FrameView anymore. Layout on the main frame can end up querying media values on such child
        frames, while detaching.
[Solution] Manually cherry-picked from https://bugs.webkit.org/show_bug.cgi?id=117754

Change-Id: Ia591e5c978279ac58c1e4cc4fa900a6ef4ba7d77