[ios] Update visibility of tab strip on trait collection changes.

Only show the tab strip in RegularxRegular trait collection.

Bug: 831978
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ibcebaeb6c2236a03f0c238f51cf00e66100d9fe5
Reviewed-on: https://chromium-review.googlesource.com/1046128
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556584}

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

@@ -164,7 +164,9 @@ const UIEdgeInsets kSearchBoxStretchInsets = {3, 3, 3, 3};
     CGFloat progress =
         [self.headerView searchFieldProgressForOffset:offset
                                        safeAreaInsets:safeAreaInsets];
-    [self.toolbarDelegate setScrollProgressForTabletOmnibox:progress];
+    if (self.isShowing) {
+      [self.toolbarDelegate setScrollProgressForTabletOmnibox:progress];
+    }
   }
 
   NSArray* constraints =