Bug 950488 - Overhaul APZ handling of overflow:hidden. r=botond

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extra : rebase_source : 6ba93899e5ac446b5c4a4cacb18574dd05c54c62
Bug 950488 - Disallow zooming for overflow:hidden frames. r=kats

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extra : rebase_source : eec649c7f5406d6781c4d703fb7bb1055c112d9e