Avoid stack allocating StyleSheetCollections.

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

Index: third_party/WebKit/Source/core/dom/ShadowTreeStyleSheetCollection.cpp
diff --git a/third_party/WebKit/Source/core/dom/ShadowTreeStyleSheetCollection.cpp b/third_party/WebKit/Source/core/dom/ShadowTreeStyleSheetCollection.cpp
index 31aabc718b704256c7345ed3e26bcc5624004a43..0c967faf147f7ce459052e493b6807e36e6e1510 100644
--- a/third_party/WebKit/Source/core/dom/ShadowTreeStyleSheetCollection.cpp
+++ b/third_party/WebKit/Source/core/dom/ShadowTreeStyleSheetCollection.cpp
@@ -66,11 +66,12 @@ void ShadowTreeStyleSheetCollection::collectStyleSheets(StyleEngine& engine, Sty
 
 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) {
@@ -78,15 +79,16 @@ void ShadowTreeStyleSheetCollection::updateActiveStyleSheets(StyleEngine& engine
             // 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