Avoid stack allocating StyleSheetCollections.

third_party/WebKit/Source/core/dom/ShadowTreeStyleSheetCollection.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
@@ skipping 48 matching lines @@
             continue;
 
         collection.appendSheetForList(sheet);
         if (candidate.canBeActivated(nullAtom))
             collection.appendActiveStyleSheet(toCSSStyleSheet(sheet));
     }
 }
 
 void ShadowTreeStyleSheetCollection::updateActiveStyleSheets(StyleEngine& engine, StyleResolverUpdateMode updateMode)
 {
-    StyleSheetCollection collection;
-    collectStyleSheets(engine, collection);
+    // StyleSheetCollection is GarbageCollected<>, allocate it on the heap.
+    StyleSheetCollection* collection = StyleSheetCollection::create();
+    collectStyleSheets(engine, *collection);
 
     StyleSheetChange change;
-    analyzeStyleSheetChange(updateMode, collection, change);
+    analyzeStyleSheetChange(updateMode, collection->activeAuthorStyleSheets(), change);
 
     if (StyleResolver* styleResolver = engine.resolver()) {
         if (change.styleResolverUpdateType != Additive) {
             // We should not destroy StyleResolver when we find any stylesheet update in a shadow tree.
             // In this case, we will reset rulesets created from style elements in the shadow tree.
             styleResolver->resetAuthorStyle(treeScope());
             styleResolver->removePendingAuthorStyleSheets(m_activeAuthorStyleSheets);
-            styleResolver->lazyAppendAuthorStyleSheets(0, collection.activeAuthorStyleSheets());
+            styleResolver->lazyAppendAuthorStyleSheets(0, collection->activeAuthorStyleSheets());
         } else {
-            styleResolver->lazyAppendAuthorStyleSheets(m_activeAuthorStyleSheets.size(), collection.activeAuthorStyleSheets());
+            styleResolver->lazyAppendAuthorStyleSheets(m_activeAuthorStyleSheets.size(), collection->activeAuthorStyleSheets());
         }
     }
     if (change.requiresFullStyleRecalc)
         toShadowRoot(treeScope().rootNode()).host().setNeedsStyleRecalc(SubtreeStyleChange, StyleChangeReasonForTracing::create(StyleChangeReason::ActiveStylesheetsUpdate));
 
-    collection.swap(*this);
+    collection->swap(*this);
+    collection->dispose();
 }
 
 } // namespace blink