Make setNeedsActiveStyleUpdate mark treescope dirty only.

third_party/WebKit/Source/core/html/HTMLLinkElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights
  * reserved.
  * Copyright (C) 2009 Rob Buis (rwlbuis@gmail.com)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 200 matching lines @@
     return;
 
   m_linkLoader->released();
 
   if (!wasConnected) {
     DCHECK(!linkStyle() || !linkStyle()->hasSheet());
     return;
   }
   document().styleEngine().removeStyleSheetCandidateNode(*this);
 
-  StyleSheet* removedSheet = sheet();
-
   if (m_link)
     m_link->ownerRemoved();
 
-  document().styleEngine().setNeedsActiveStyleUpdate(removedSheet,
-                                                     FullStyleUpdate);
+  document().styleEngine().setNeedsActiveStyleUpdate(treeScope());

[meade_UTC10, 2016/12/05 23:57:22]

It looks like the treeScope is not yet registered in the StyleEngine, which is
why the DCHECK I commented on is getting hit. Do you need to call 
document().styleEngine().styleSheetsForStyleSheetList(treeScope()) or 
move this call to before line 219?

[rune, 2016/12/06 14:53:22]

It's the wrong treeScope(), we should have found the right ShadowRoot treeScope
for the removeStyleSheetCandidateNode(). I don't think we even need the
setNeedsActiveStyleUpdate call here since removeStyleSheetCandidateNode will do
that for us.

Anyway, needed to fix https://crbug.com/671596 first:

https://codereview.chromium.org/2551973003/

+  // TODO(rune@opera.com): resolverChanged() can be removed once stylesheet
+  // updates are async. https://crbug.com/567021
+  document().styleEngine().resolverChanged(FullStyleUpdate);
 }
 
 void HTMLLinkElement::finishParsingChildren() {
   m_createdByParser = false;
   HTMLElement::finishParsingChildren();
 }
 
 bool HTMLLinkElement::styleSheetIsLoading() const {
   return linkStyle() && linkStyle()->styleSheetIsLoading();
 }
@@ skipping 122 matching lines @@
   LinkLoaderClient::trace(visitor);
   DOMTokenListObserver::trace(visitor);
 }
 
 DEFINE_TRACE_WRAPPERS(HTMLLinkElement) {
   visitor->traceWrappers(m_relList);
   HTMLElement::traceWrappers(visitor);
 }
 
 }  // namespace blink