Fix for nullptr crash in SpatNav

No test since I don't know how to setup this situation. However, it's
safe to just avoid scrolling in this case.

(cherry picked from commit 3438927)

Bug: 970164
Change-Id: Icbae6b631e2f80e9e6493825fefb0f10fab974f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1645792
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#667579}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1662817
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/branch-heads/3770@{#1031}
Cr-Branched-From: a9eee1c-refs/heads/master@{#652427}

third_party/blink/renderer/core/page/spatial_navigation.cc

@@ -289,7 +289,9 @@ bool ScrollInDirection(Node* container, SpatialNavigationDirection direction) {
   // CanScrollInDirection(). Regular arrow-key scrolling (without
   // --enable-spatial-navigation) already uses smooth scrolling by default.
   ScrollableArea* scroller = ScrollableAreaFor(container);
-  DCHECK(scroller);
+  if (!scroller)
+    return false;
+
   scroller->ScrollBy(ScrollOffset(dx, dy), kUserScroll);
   return true;
 }