[ios] Stops observing FeedView when its VC is removed.

Bug: 1085419
Change-Id: I3d2dee5f2241aa244cf1bdc2f5d7d8cf7005908d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2365095Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799915}

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

@@ -111,6 +111,7 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
 }
 
 - (void)dealloc {
+  [self.feedView removeObserver:self forKeyPath:@"contentSize"];
   [self.discoverFeedVC willMoveToParentViewController:nil];
   [self.discoverFeedVC.view removeFromSuperview];
   [self.discoverFeedVC removeFromParentViewController];
@@ -419,8 +420,10 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
     UIViewController* newFeedViewController = discoverFeedItem.discoverFeed;
 
     if (newFeedViewController != self.discoverFeedVC) {
-      // If previous VC is not nil, remove it from the view hierarchy.
+      // If previous VC is not nil, remove it from the view hierarchy and stop
+      // osberving its feedView.
       if (self.discoverFeedVC) {
+        [self.feedView removeObserver:self forKeyPath:@"contentSize"];
         [self.discoverFeedVC willMoveToParentViewController:nil];
         [self.discoverFeedVC.view removeFromSuperview];
         [self.discoverFeedVC removeFromParentViewController];