Return ActiveSheetsChanged when rulesets change in common prefix.

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

Index: third_party/WebKit/Source/core/css/ActiveStyleSheetsTest.cpp
diff --git a/third_party/WebKit/Source/core/css/ActiveStyleSheetsTest.cpp b/third_party/WebKit/Source/core/css/ActiveStyleSheetsTest.cpp
index 1cd52d06d0b5a4b10a4d9b6a833a7c7a20bfb4af..1007d24c51d26270f4ef417d1313d44a02639648 100644
--- a/third_party/WebKit/Source/core/css/ActiveStyleSheetsTest.cpp
+++ b/third_party/WebKit/Source/core/css/ActiveStyleSheetsTest.cpp
@@ -328,6 +328,23 @@ TEST_F(ActiveStyleSheetsTest, CompareActiveStyleSheets_ReorderedImportSheets) {
   EXPECT_EQ(0u, changedRuleSets.size());
 }
 
+TEST_F(ActiveStyleSheetsTest, CompareActiveStyleSheets_DisableAndAppend) {
+  ActiveStyleSheetVector oldSheets;
+  ActiveStyleSheetVector newSheets;
+  HeapHashSet<Member<RuleSet>> changedRuleSets;
+
+  CSSStyleSheet* sheet1 = createSheet();
+  CSSStyleSheet* sheet2 = createSheet();
+
+  oldSheets.push_back(std::make_pair(sheet1, &sheet1->contents()->ruleSet()));
+  newSheets.push_back(std::make_pair(sheet1, nullptr));
+  newSheets.push_back(std::make_pair(sheet2, &sheet2->contents()->ruleSet()));
+
+  EXPECT_EQ(ActiveSheetsChanged,
+            compareActiveStyleSheets(oldSheets, newSheets, changedRuleSets));
+  EXPECT_EQ(2u, changedRuleSets.size());
+}
+
 TEST_F(ApplyRulesetsTest, AddUniversalRuleToDocument) {
   document().view()->updateAllLifecyclePhases();