Add header for the Content Suggestions

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/ui/content_suggestions/content_suggestions_view_controller.h"
 
 #include "base/mac/foundation_util.h"
 #import "ios/chrome/browser/ui/collection_view/cells/MDCCollectionViewCell+Chrome.h"
 #import "ios/chrome/browser/ui/collection_view/cells/collection_view_item.h"
 #import "ios/chrome/browser/ui/collection_view/collection_view_model.h"
@@ skipping 166 matching lines @@
 
 - (BOOL)collectionView:(nonnull UICollectionView*)collectionView
     shouldHideItemBackgroundAtIndexPath:(nonnull NSIndexPath*)indexPath {
   if ([self.collectionUpdater
           shouldUseCustomStyleForSection:indexPath.section]) {
     return YES;
   }
   return NO;
 }
 
+- (BOOL)collectionView:(UICollectionView*)collectionView
+    shouldHideHeaderBackgroundForSection:(NSInteger)section {
+  return YES;
+}
+
 - (CGFloat)collectionView:(UICollectionView*)collectionView
     cellHeightAtIndexPath:(NSIndexPath*)indexPath {
   CollectionViewItem* item =
       [self.collectionViewModel itemAtIndexPath:indexPath];
   UIEdgeInsets inset = [self collectionView:collectionView
                                      layout:collectionView.collectionViewLayout
                      insetForSectionAtIndex:indexPath.section];
 
   return [MDCCollectionViewCell
       cr_preferredHeightForWidth:CGRectGetWidth(collectionView.bounds) -
@@ skipping 58 matching lines @@
   ContentSuggestionsArticleItem* articleItem =
       base::mac::ObjCCastStrict<ContentSuggestionsArticleItem>(touchedItem);
 
   [self.suggestionCommandHandler
       displayContextMenuForArticle:articleItem
                            atPoint:touchLocation
                        atIndexPath:touchedItemIndexPath];
 }
 
 @end