Verify that constant vectors aren't invalidated during iteration.

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All
  * rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
@@ skipping 311 matching lines @@
 void RuleSet::addRulesFromSheet(StyleSheetContents* sheet,
                                 const MediaQueryEvaluator& medium,
                                 AddRuleFlags addRuleFlags) {
   TRACE_EVENT0("blink", "RuleSet::addRulesFromSheet");
 
   DCHECK(sheet);
 
   const HeapVector<Member<StyleRuleImport>>& importRules = sheet->importRules();
   for (unsigned i = 0; i < importRules.size(); ++i) {
     StyleRuleImport* importRule = importRules[i].get();
+    // TODO(sof): CHECK() added for crbug.com/699269 diagnosis, remove sooner.
+    CHECK_EQ(importRules.data(), sheet->importRules().data());
     if (importRule->styleSheet() &&
         (!importRule->mediaQueries() ||
          medium.eval(importRule->mediaQueries(),
                      &m_features.viewportDependentMediaQueryResults(),
                      &m_features.deviceDependentMediaQueryResults())))
       addRulesFromSheet(importRule->styleSheet(), medium, addRuleFlags);
   }
 
   addChildRules(sheet->childRules(), medium, addRuleFlags);
 }
@@ skipping 85 matching lines @@
 }
 
 #ifndef NDEBUG
 void RuleSet::show() const {
   for (const auto& rule : m_allRules)
     rule.selector().show();
 }
 #endif
 
 }  // namespace blink