[iOS] Fix thumb strip thumbnails overlapping status bar

The major issue here was that the height calculation in
horizontal_layout.mm wasn't taking the collection view's contentInsets
into account when calculating the sectionInset. This meant that the
resulting layout was too large. Specifically, the vertical sectionInsets
+ contentInset + item height ended up larger than the collection view's
height, causing the layout to be incorrect.

Fixed: 1155601
Change-Id: Ie9594fec2762239963bc90c41bf07f8caa42bf83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587038Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#836196}

ios/chrome/browser/ui/tab_switcher/tab_grid/grid/horizontal_layout.mm

@@ -27,12 +27,14 @@
   [super prepareLayout];
 
   self.itemSize = kGridCellSizeSmall;
-  CGFloat height = CGRectGetHeight(self.collectionView.bounds);
   CGFloat spacing = kGridLayoutLineSpacingCompactCompactLimitedWidth;
-  CGFloat topInset = spacing + kGridCellSelectionRingGapWidth +
-                     kGridCellSelectionRingTintWidth;
+  // Use the height of the available content area.
+  CGFloat height = CGRectGetHeight(UIEdgeInsetsInsetRect(
+      self.collectionView.bounds, self.collectionView.contentInset));
+  CGFloat topInset =
+      kGridCellSelectionRingGapWidth + kGridCellSelectionRingTintWidth;
   self.sectionInset = UIEdgeInsets{
-      topInset, spacing, height - self.itemSize.height - 2 * topInset, spacing};
+      topInset, spacing, height - self.itemSize.height - topInset, spacing};
   self.minimumLineSpacing = kGridLayoutLineSpacingRegularRegular;
 }