Oilpan: fix build after r201639.

Source/core/css/ElementRuleCollector.h

Index: Source/core/css/ElementRuleCollector.h
diff --git a/Source/core/css/ElementRuleCollector.h b/Source/core/css/ElementRuleCollector.h
index d8bc12f71eed55c9b8d611bed6a15d0db96086c6..67b44a7a65a370dd638b3fddce64da47567f496f 100644
--- a/Source/core/css/ElementRuleCollector.h
+++ b/Source/core/css/ElementRuleCollector.h
@@ -62,7 +62,11 @@ public:
     }
 
 private:
-    RawPtrWillBeMember<const RuleData> m_ruleData;
+    // FIXME: Oilpan: RuleData is in the oilpan heap and this pointer
+    // really should be traced. However, RuleData objects are
+    // allocated inside larger TerminatedArray objects and we cannot
+    // trace a raw rule data pointer at this point.
+    const RuleData* m_ruleData;
     unsigned m_specificity;
     uint64_t m_position;
     RawPtrWillBeMember<const CSSStyleSheet> m_parentStyleSheet;