Don't access body LayoutObject marked for reattach.

With the work for unified ComputedStyle storage on element, found that
we are accessing the LayoutObject from the old layout tree when the
viewport defining element changes and body is marked for re-attach.

TEST=external/wpt/cssom-view/scrollingElement-*

Change-Id: I5b8d8f8291d05d058af61779992c24381f3edba0
Reviewed-on: https://chromium-review.googlesource.com/c/1356479
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613032}

third_party/blink/renderer/core/dom/document.cc

@@ -2380,6 +2380,8 @@ void Document::ViewportDefiningElementDidChange() {
   HTMLBodyElement* body = FirstBodyElement();
   if (!body)
     return;
+  if (body->NeedsReattachLayoutTree())
+    return;
   LayoutObject* layout_object = body->GetLayoutObject();
   if (layout_object && layout_object->IsLayoutBlock()) {
     // When the overflow style for documentElement changes to or from visible,