Don't retain next_sibling being re-attached or skipped.

We ended keeping a deleted LayoutObject as part of whitespace re-
attachement because the previous next_sibling was detached at the next
iteration. Reset the next_sibling for each iteration.

I have not been able to reproduce the issue locally with the clusterfuzz
cases, and not figured out how to construct a case which causes this to
happen, so this is a speculative fix.

Bug: 977616
Change-Id: I31c840096897b757aaeccb3d8085ad57cf6b3060
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1674019Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671810}

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

@@ -139,6 +139,8 @@ void WhitespaceAttacher::ReattachWhitespaceSiblings(
                !sibling_layout_object->IsFloatingOrOutOfFlowPositioned()) {
       break;
     }
+    context.next_sibling_valid = false;
+    context.next_sibling = nullptr;
   }
   SetLastTextNode(nullptr);
 }