Removes superfluous lines in ContentSuggestions

The separators of the cells and headers where displayed even if they
had no background.
This CL removes those lines.

Bug: 740565
Change-Id: If6e2ab6225eec5e7cfc0112c8cff475530c479ed
Reviewed-on: https://chromium-review.googlesource.com/602228Reviewed-by: Jean-François Geyelin <jif@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492004}

ios/chrome/browser/ui/content_suggestions/content_suggestions_view_controller.mm

@@ -392,6 +392,17 @@ BOOL ShouldCellsBeFullWidth(UITraitCollection* collection) {
   return [item cellHeightForWidth:width];
 }
 
+- (BOOL)collectionView:(UICollectionView*)collectionView
+    shouldHideItemSeparatorAtIndexPath:(NSIndexPath*)indexPath {
+  return [self collectionView:collectionView
+      shouldHideItemBackgroundAtIndexPath:indexPath];
+}
+
+- (BOOL)collectionView:(UICollectionView*)collectionView
+    shouldHideHeaderSeparatorForSection:(NSInteger)section {
+  return YES;
+}
+
 #pragma mark - MDCCollectionViewEditingDelegate
 
 - (BOOL)collectionViewAllowsSwipeToDismissItem: