Allow active sheets to have nullptr for RuleSet.

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/css/ActiveStyleSheets.h"
 
 #include "core/css/CSSStyleSheet.h"
 #include "core/css/RuleSet.h"
 
 namespace blink {
 
 ActiveSheetsChange compareActiveStyleSheets(
     const ActiveStyleSheetVector& oldStyleSheets,
     const ActiveStyleSheetVector& newStyleSheets,
     HeapVector<Member<RuleSet>>& changedRuleSets) {
   unsigned newStyleSheetCount = newStyleSheets.size();
   unsigned oldStyleSheetCount = oldStyleSheets.size();
 
   unsigned minCount = std::min(newStyleSheetCount, oldStyleSheetCount);
   unsigned index = 0;
 
   // Walk the common prefix of stylesheets. If the stylesheet rules were
   // modified since last time, add them to the list of changed rulesets.
   for (; index < minCount &&
          newStyleSheets[index].first == oldStyleSheets[index].first;
        index++) {
     if (newStyleSheets[index].second == oldStyleSheets[index].second)
       continue;
 
-    changedRuleSets.append(newStyleSheets[index].second);
-    changedRuleSets.append(oldStyleSheets[index].second);
+    if (newStyleSheets[index].second)
+      changedRuleSets.append(newStyleSheets[index].second);
+    if (oldStyleSheets[index].second)
+      changedRuleSets.append(oldStyleSheets[index].second);
   }
 
   if (index == oldStyleSheetCount) {
     if (index == newStyleSheetCount)
       return changedRuleSets.size() ? ActiveSheetsChanged
                                     : NoActiveSheetsChanged;
 
     // Sheets added at the end.
-    for (; index < newStyleSheetCount; index++)
-      changedRuleSets.append(newStyleSheets[index].second);
-    return ActiveSheetsAppended;
+    for (; index < newStyleSheetCount; index++) {
+      if (newStyleSheets[index].second)
+        changedRuleSets.append(newStyleSheets[index].second);
+    }
+    return changedRuleSets.size() ? ActiveSheetsAppended
+                                  : NoActiveSheetsChanged;
   }
 
   if (index == newStyleSheetCount) {
     // Sheets removed from the end.
-    for (; index < oldStyleSheetCount; index++)
-      changedRuleSets.append(oldStyleSheets[index].second);
-    return ActiveSheetsChanged;
+    for (; index < oldStyleSheetCount; index++) {
+      if (oldStyleSheets[index].second)
+        changedRuleSets.append(oldStyleSheets[index].second);
+    }
+    return changedRuleSets.size() ? ActiveSheetsChanged : NoActiveSheetsChanged;
   }
 
   DCHECK(index < oldStyleSheetCount && index < newStyleSheetCount);
 
   // Both the new and old active stylesheet vectors have stylesheets following
   // the common prefix. Figure out which were added or removed by sorting the
   // merged vector of old and new sheets.
 
   ActiveStyleSheetVector mergedSorted;
   mergedSorted.reserveCapacity(oldStyleSheetCount + newStyleSheetCount -
                                2 * index);
   mergedSorted.appendRange(oldStyleSheets.begin() + index,
                            oldStyleSheets.end());
   mergedSorted.appendRange(newStyleSheets.begin() + index,
                            newStyleSheets.end());
 
   std::sort(mergedSorted.begin(), mergedSorted.end());
 
   auto mergedIterator = mergedSorted.begin();
   while (mergedIterator != mergedSorted.end()) {
     const auto& sheet1 = *mergedIterator++;
     if (mergedIterator == mergedSorted.end() ||
         (*mergedIterator).first != sheet1.first) {
       // Sheet either removed or inserted.
-      changedRuleSets.append(sheet1.second);
+      if (sheet1.second)
+        changedRuleSets.append(sheet1.second);
       continue;
     }
 
     // Sheet present in both old and new.
     const auto& sheet2 = *mergedIterator++;
 
     if (sheet1.second == sheet2.second)
       continue;
 
     // Active rules for the given stylesheet changed.
     // DOM, CSSOM, or media query changes.
-    changedRuleSets.append(sheet1.second);
-    changedRuleSets.append(sheet2.second);
+    if (sheet1.second)
+      changedRuleSets.append(sheet1.second);
+    if (sheet2.second)
+      changedRuleSets.append(sheet2.second);
   }
-  return ActiveSheetsChanged;
+  return changedRuleSets.size() ? ActiveSheetsChanged : NoActiveSheetsChanged;
 }
 
 }  // namespace blink