Add containerContentOffset to IGListCollectionContext

Summary:
**In this diff**

Add `containerContentOffset` to `IGListCollectionContext`
- In this stack, I check this value from the product tile and content tile section controllers to determine how far the user has scrolled down the containing scroll view

**In this stack**

Add `scroll_logging_info` to product card and content tile events to track scroll depth on the Shop Tab homepage
- Project plan: https://fb.quip.com/mF9qAnGPE8CI
- Logging spec: https://fb.quip.com/6wp7AO63d8Wb#LPWACADjpNK

Differential Revision: D27793071

fbshipit-source-id: 22123511b0f85fd636d82981671ccda0b0d7db7d

Source/IGListKit/IGListAdapter.m

@@ -943,6 +943,10 @@
     return UIEdgeInsetsInsetRect(collectionView.bounds, collectionView.ig_contentInset).size;
 }
 
+- (CGPoint)containerContentOffset {
+    return self.collectionView.contentOffset;
+}
+
 - (IGListCollectionScrollingTraits)scrollingTraits {
     UICollectionView *collectionView = self.collectionView;
     return (IGListCollectionScrollingTraits) {

Source/IGListKit/IGListCollectionContext.h

@@ -42,6 +42,11 @@ NS_SWIFT_NAME(ListCollectionContext)
  */
 @property (nonatomic, readonly) CGSize insetContainerSize;
 
+/**
+ The content offset of the collection view.
+ */
+@property (nonatomic, readonly) CGPoint containerContentOffset;
+
 /**
  The current scrolling traits of the underlying collection view.
  */