Adding/Removing same class multiple times schedules same invalidation set.

third_party/WebKit/Source/core/css/invalidation/StyleInvalidator.cpp

Index: third_party/WebKit/Source/core/css/invalidation/StyleInvalidator.cpp
diff --git a/third_party/WebKit/Source/core/css/invalidation/StyleInvalidator.cpp b/third_party/WebKit/Source/core/css/invalidation/StyleInvalidator.cpp
index 6cd632a3bcb8d1de8cc3cd032d7a8a92c4c5a921..67ab7acaf02a820acb65485d6832ca01a08f0221 100644
--- a/third_party/WebKit/Source/core/css/invalidation/StyleInvalidator.cpp
+++ b/third_party/WebKit/Source/core/css/invalidation/StyleInvalidator.cpp
@@ -69,8 +69,11 @@ void StyleInvalidator::scheduleInvalidationSetsForElement(const InvalidationList
 
     PendingInvalidations& pendingInvalidations = ensurePendingInvalidations(element);
     if (element.nextSibling()) {
-        for (auto& invalidationSet : invalidationLists.siblings)
+        for (auto& invalidationSet : invalidationLists.siblings) {
+            if (pendingInvalidations.siblings().contains(invalidationSet))
+                continue;
             pendingInvalidations.siblings().append(invalidationSet);
+        }
     }
 
     if (!requiresDescendantInvalidation)
@@ -78,8 +81,11 @@ void StyleInvalidator::scheduleInvalidationSetsForElement(const InvalidationList
 
     for (auto& invalidationSet : invalidationLists.descendants) {
         ASSERT(!invalidationSet->wholeSubtreeInvalid());
-        if (!invalidationSet->isEmpty())
-            pendingInvalidations.descendants().append(invalidationSet);
+        if (invalidationSet->isEmpty())
+            continue;
+        if (pendingInvalidations.descendants().contains(invalidationSet))
+            continue;
+        pendingInvalidations.descendants().append(invalidationSet);
     }
 }