[iOS] Fix NTP frame

This CL fixes the frame computation for the NTP, taking into account
the BrowserContainerFullscreen experiment.

Bug: 918435
Change-Id: I55e31b86262c3b30ae3a51029dba2336929076a5
Reviewed-on: https://chromium-review.googlesource.com/c/1392956
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619940}

ios/chrome/browser/ui/browser_view_controller.mm

@@ -2513,6 +2513,10 @@ NSString* const kBrowserViewControllerSnackbarCategory =
   NewTabPageTabHelper* NTPHelper = NewTabPageTabHelper::FromWebState(webState);
   if (!NTPHelper || !NTPHelper->IsActive())
     return CGRectZero;
+  if (base::FeatureList::IsEnabled(
+          web::features::kBrowserContainerFullscreen) &&
+      !IsRegularXRegularSizeClass())
+    return self.contentArea.bounds;
   // NTP expects to be laid out behind the bottom toolbar.  It uses
   // |contentInset| to push content above the toolbar.
   UIEdgeInsets viewportInsets = [self viewportInsetsForView:self.contentArea];