Bug 915985 - Refactoring to introduce a ZoomConstraints class. r=botond a=1.3+
Bug 957790 - Remove a no-longer-used pref to disable async zooming in the APZ. r=jimm a=1.3+
Bug 957790 - Initialize the APZC with zooming disabled until we know for sure if it is allowed. r=botond,jimm a=1.3+

On Metro we don't support the meta-viewport tag yet, so we need to re-enable
zooming there.