scrollView在iOS11新增的两个属性：adjustContentInset 和 contentInsetAdjustmentBehavior。
/* When contentInsetAdjustmentBehavior allows, UIScrollView may incorporate
its safeAreaInsets into the adjustedContentInset.
*/
@property(nonatomic, readonly) UIEdgeInsets adjustedContentInset API_AVAILABLE(ios(11.0),tvos(11.0));
adjustContentInset表示contentView.frame.origin偏移了scrollview.frame.origin多少；是系统计算得来的，计算方式由contentInsetAdjustmentBehavior决定。有以下几种计算方式：
UIScrollViewContentInsetAdjustmentAutomatic：如果
scrollview在一个automaticallyAdjustsScrollViewContentInset = YES的controller上，并且这个Controller包含在一个navigation controller中，这种情况下会设置在top & bottom上adjustedContentInset = safeAreaInset + contentInset不管是否滚动。其他情况下与UIScrollViewContentInsetAdjustmentScrollableAxes相同UIScrollViewContentInsetAdjustmentScrollableAxes: 在可滚动方向上
adjustedContentInset = safeAreaInset + contentInset，在不可滚动方向上adjustedContentInset = contentInset；依赖于scrollEnabled和alwaysBounceHorizontal / vertical = YES，scrollEnabled默认为yes，所以大多数情况下，计算方式还是adjustedContentInset = safeAreaInset + contentInsetUIScrollViewContentInsetAdjustmentNever:
adjustedContentInset = contentInsetUIScrollViewContentInsetAdjustmentAlways:
adjustedContentInset = safeAreaInset + contentInset
当contentInsetAdjustmentBehavior设置为UIScrollViewContentInsetAdjustmentNever的时候，adjustContentInset值不受SafeAreaInset值的影响。
