[ios Checks ContentSuggestions KVO flag before any KVO change

Bug: 1122031
Change-Id: Ie2733764f57d824cb9bf61155c69ddb248e4c704
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2468715Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816793}

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

@@ -125,10 +125,7 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
 }
 
 - (void)dealloc {
-  if (self.observingDiscoverFeedHeight) {
-    [self.feedView removeObserver:self forKeyPath:@"contentSize"];
-    self.observingDiscoverFeedHeight = NO;
-  }
+  [self removeContentSizeKVO];
   if (self.discoverFeedVC.parentViewController) {
     [self.discoverFeedVC willMoveToParentViewController:nil];
     [self.discoverFeedVC.view removeFromSuperview];
@@ -442,8 +439,7 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
       // 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.observingDiscoverFeedHeight = NO;
+        [self removeContentSizeKVO];
         [self.discoverFeedVC willMoveToParentViewController:nil];
         [self.discoverFeedVC.view removeFromSuperview];
         [self.discoverFeedVC removeFromParentViewController];
@@ -462,11 +458,7 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
             self.feedView = static_cast<UICollectionView*>(view);
           }
         }
-        [self.feedView addObserver:self
-                        forKeyPath:@"contentSize"
-                           options:0
-                           context:nil];
-        self.observingDiscoverFeedHeight = YES;
+        [self addContentSizeKVO];
         self.discoverFeedVC = newFeedViewController;
         return cell;
       }
@@ -841,6 +833,25 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
 
 #pragma mark - Private
 
+// Adds KVO observing for the feedView contentSize if there is not one already.
+- (void)addContentSizeKVO {
+  if (!self.observingDiscoverFeedHeight) {
+    [self.feedView addObserver:self
+                    forKeyPath:@"contentSize"
+                       options:0
+                       context:nil];
+    self.observingDiscoverFeedHeight = YES;
+  }
+}
+
+// Removes KVO observing for the feedView contentSize if one exists.
+- (void)removeContentSizeKVO {
+  if (self.observingDiscoverFeedHeight) {
+    [self.feedView removeObserver:self forKeyPath:@"contentSize"];
+    self.observingDiscoverFeedHeight = NO;
+  }
+}
+
 - (void)handleLongPress:(UILongPressGestureRecognizer*)gestureRecognizer {
   if (self.editor.editing ||
       gestureRecognizer.state != UIGestureRecognizerStateBegan) {