Masonry 在iOS 11中新增属性：
@property (nonatomic, strong, readonly) MASViewAttribute *mas_safeAreaLayoutGuide NS_AVAILABLE_IOS(11.0);
@property (nonatomic, strong, readonly) MASViewAttribute *mas_safeAreaLayoutGuideLeading NS_AVAILABLE_IOS(11.0);
@property (nonatomic, strong, readonly) MASViewAttribute *mas_safeAreaLayoutGuideTrailing NS_AVAILABLE_IOS(11.0);
@property (nonatomic, strong, readonly) MASViewAttribute *mas_safeAreaLayoutGuideLeft NS_AVAILABLE_IOS(11.0);
@property (nonatomic, strong, readonly) MASViewAttribute *mas_safeAreaLayoutGuideRight NS_AVAILABLE_IOS(11.0);
@property (nonatomic, strong, readonly) MASViewAttribute *mas_safeAreaLayoutGuideTop NS_AVAILABLE_IOS(11.0);
@property (nonatomic, strong, readonly) MASViewAttribute *mas_safeAreaLayoutGuideBottom NS_AVAILABLE_IOS(11.0);
@property (nonatomic, strong, readonly) MASViewAttribute *mas_safeAreaLayoutGuideWidth NS_AVAILABLE_IOS(11.0);
@property (nonatomic, strong, readonly) MASViewAttribute *mas_safeAreaLayoutGuideHeight NS_AVAILABLE_IOS(11.0);
@property (nonatomic, strong, readonly) MASViewAttribute *mas_safeAreaLayoutGuideCenterX NS_AVAILABLE_IOS(11.0);
@property (nonatomic, strong, readonly) MASViewAttribute *mas_safeAreaLayoutGuideCenterY NS_AVAILABLE_IOS(11.0);
用法：
[self.containerView mas_makeConstraints:^(MASConstraintMaker *make) {
if (@available(iOS 11.0, *)) {
make.edges.equalTo(self.view.mas_safeAreaLayoutGuide);
} else {
make.edges.equalTo(self.view);
}
}];
