Fix selector namespace prefix resolution.

third_party/WebKit/Source/core/css/CSSStyleRule.cpp

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * (C) 2002-2003 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2002, 2005, 2006, 2008, 2012 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 77 matching lines @@
     ASSERT(!selectorTextCache().contains(this));
     String text = generateSelectorText();
     selectorTextCache().set(this, text);
     setHasCachedSelectorText(true);
     return text;
 }
 
 void CSSStyleRule::setSelectorText(const String& selectorText)
 {
     CSSParserContext context(parserContext(), 0);
-    CSSSelectorList selectorList = CSSParser::parseSelector(context, selectorText);
+    CSSSelectorList selectorList = CSSParser::parseSelector(context, parentStyleSheet()->contents(), selectorText);

[Timothy Loh, 2016/01/25 00:30:47]

You probably need to null-check parentStyleSheet (here and CSSPageRule), it ends
up null in some cases (fast/dom/css-delete-doc.html).

[rune, 2016/01/25 08:58:48]

Done.

     if (!selectorList.isValid())
         return;
 
     CSSStyleSheet::RuleMutationScope mutationScope(this);
 
     m_styleRule->wrapperAdoptSelectorList(std::move(selectorList));
 
     if (hasCachedSelectorText()) {
         selectorTextCache().remove(this);
         setHasCachedSelectorText(false);
@@ skipping 22 matching lines @@
 }
 
 DEFINE_TRACE(CSSStyleRule)
 {
     visitor->trace(m_styleRule);
     visitor->trace(m_propertiesCSSOMWrapper);
     CSSRule::trace(visitor);
 }
 
 } // namespace blink