[ios] Moves FeedView KVO to setter.

This way we make sure to stop/start observing whenever a new FeedView
has been set, making sure the calls are balanced for all FeedViews.

Bug: 1122031
Change-Id: If6392e5b3e9692141b441162665aebd95ddd4b06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490024Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819522}

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

@@ -436,10 +436,9 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
     UIViewController* newFeedViewController = discoverFeedItem.discoverFeed;
 
     if (newFeedViewController != self.discoverFeedVC) {
-      // If previous VC is not nil, remove it from the view hierarchy and stop
-      // osberving its feedView.
+      // If previous VC is not nil, remove it from the view hierarchy.
       if (self.discoverFeedVC) {
-        [self removeContentSizeKVO];
+        self.feedView = nil;
         [self.discoverFeedVC willMoveToParentViewController:nil];
         [self.discoverFeedVC.view removeFromSuperview];
         [self.discoverFeedVC removeFromParentViewController];
@@ -458,7 +457,6 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
             self.feedView = static_cast<UICollectionView*>(view);
           }
         }
-        [self addContentSizeKVO];
         self.discoverFeedVC = newFeedViewController;
         return cell;
       }
@@ -833,6 +831,15 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
 
 #pragma mark - Private
 
+// |self.feedView| setter.
+- (void)setFeedView:(UICollectionView*)feedView {
+  if (feedView != _feedView) {
+    [self removeContentSizeKVO];
+    _feedView = feedView;
+    [self addContentSizeKVO];
+  }
+}
+
 // Adds KVO observing for the feedView contentSize if there is not one already.
 - (void)addContentSizeKVO {
   if (!self.observingDiscoverFeedHeight) {