Fix constraints for NTP fake omnibox

This CL fixes the constraints applied to the fake omnibox of the NTP
when it is scrolled up.

Bug: 791486
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Iec1ad1069caebcd7c78a78f2ed155be011050374
Reviewed-on: https://chromium-review.googlesource.com/805246Reviewed-by: Elodie Banel <lod@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521300}

ios/chrome/browser/ui/ntp/new_tab_page_header_view.mm

@@ -146,7 +146,7 @@
       ntp_header::kToolbarHeight - content_suggestions::kSearchFieldHeight;
 
   widthConstraint.constant = searchFieldNormalWidth - 2 * maxXInset * percent;
-  topMarginConstraint.constant = -content_suggestions::searchFieldTopMargin() +
+  topMarginConstraint.constant = -content_suggestions::searchFieldTopMargin() -
                                  ntp_header::kMaxTopMarginDiff * percent;
   heightConstraint.constant =
       content_suggestions::kSearchFieldHeight + maxHeightDiff * percent;