Don't add to uncommonAttributeRules for non-sharable cases.

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

Index: third_party/WebKit/Source/core/css/RuleSet.cpp
diff --git a/third_party/WebKit/Source/core/css/RuleSet.cpp b/third_party/WebKit/Source/core/css/RuleSet.cpp
index 42962a163bd9683a5086e89d1dfbedfc6b8382d0..663371ae1e0e3f8130f6b1480ed4385491360480 100644
--- a/third_party/WebKit/Source/core/css/RuleSet.cpp
+++ b/third_party/WebKit/Source/core/css/RuleSet.cpp
@@ -78,6 +78,12 @@ static bool containsUncommonAttributeSelector(const CSSSelector& selector)
             return true;
         if (selectorListContainsUncommonAttributeSelector(current))
             return true;
+        if (current->relationIsAffectedByPseudoContent()
+            || current->pseudoType() == CSSSelector::PseudoHost
+            || current->pseudoType() == CSSSelector::PseudoHostContext
+            || current->pseudoType() == CSSSelector::PseudoSlotted) {
+            return false;
+        }
         if (current->relation() != CSSSelector::SubSelector) {
             current = current->tagHistory();
             break;