Add handler for scrollToTop behaviour in NTP

Previously, scrollToTop while the omnibox is focused would scroll to the top and 2 omnibox would be visible.

Now, first unfocus omnibox, then allow scrollToTop.

Before: https://drive.google.com/file/d/1zn69R672sRTRe6GG3s4Mc7WVo2Q6G4xo/view?usp=sharing
After (Patchset 1-6) : https://drive.google.com/file/d/1fiiZZBwWfXb7gb66dCfOwW6yGSBemayi/view?usp=sharing
After stuttering fix (Patchset 7+) : https://drive.google.com/file/d/1A2FavkilxFqFVFjh2A5wN0SATsrMENGx/view?usp=sharing

Change-Id: I3c026bb800a243ef0bab150cfac4f696bbb14c40
Bug: 965473
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2201896
Commit-Queue: Maxime Charland <mcharland@google.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770853}

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

@@ -214,6 +214,10 @@ initWithCollectionController:
   [self.headerController updateConstraints];
 }
 
+- (void)resetPreFocusOffset {
+  self.collectionShiftingOffset = 0;
+}
+
 - (void)unfocusOmnibox {
   [self.headerController unfocusOmnibox];
 }

ios/chrome/browser/ui/content_suggestions/content_suggestions_header_synchronizing.h

@@ -22,6 +22,9 @@
 // Updates the fake omnibox to adapt to the current orientation.
 - (void)updateFakeOmniboxOnNewWidth:(CGFloat)width;
 
+// Sets the amount of scrolling needed when unfocusing the omnibox to 0.
+- (void)resetPreFocusOffset;
+
 // Unfocuses the omnibox.
 - (void)unfocusOmnibox;
 

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

@@ -564,6 +564,16 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
       scrollView.contentOffset.y >= [self.headerSynchronizer pinnedOffsetY];
 }
 
+- (BOOL)scrollViewShouldScrollToTop:(UIScrollView*)scrollView {
+  // User has tapped the status bar to scroll to the top.
+  // Prevent scrolling back to pre-focus state, making sure we don't have
+  // two scrolling animations running at the same time.
+  [self.headerSynchronizer resetPreFocusOffset];
+  // Unfocus omnibox without scrolling back.
+  [self.headerSynchronizer unfocusOmnibox];
+  return YES;
+}
+
 - (void)scrollViewWillBeginDragging:(UIScrollView*)scrollView {
   [self.overscrollActionsController scrollViewWillBeginDragging:scrollView];
 }