On iPhone transition to "passive" does not happen on blur event

[naruaway]

On iPhone (tested in iOS simulator 12.2), document.hasFocus() returns true even during onblur handler.
Because of this, active -> passive transition is not happening when going to "list of tabs" view.

This fix is overriding document.hasFocus() while handling onblur so that it will be always treated as "transitioning to passive"

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[naruaway]

@googlebot I signed it!

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