Oilpan: add reqd tracing of ImportedStyleSheetClients.

third_party/WebKit/Source/core/css/StyleRuleImport.h

Index: third_party/WebKit/Source/core/css/StyleRuleImport.h
diff --git a/third_party/WebKit/Source/core/css/StyleRuleImport.h b/third_party/WebKit/Source/core/css/StyleRuleImport.h
index bcea473cbce380e6af348ea2d2de4b30ced45b71..2c795e15548eeec8d7cf02707dfcb83e4ddec64e 100644
--- a/third_party/WebKit/Source/core/css/StyleRuleImport.h
+++ b/third_party/WebKit/Source/core/css/StyleRuleImport.h
@@ -68,8 +68,14 @@ private:
             m_ownerRule->setCSSStyleSheet(href, baseURL, charset, sheet);
         }
         String debugName() const override { return "ImportedStyleSheetClient"; }
+
+        DEFINE_INLINE_TRACE()
+        {
+            visitor->trace(m_ownerRule);
+        }
+
     private:
-        StyleRuleImport* m_ownerRule;
+        RawPtrWillBeMember<StyleRuleImport> m_ownerRule;
     };
 
     void setCSSStyleSheet(const String& href, const KURL& baseURL, const String& charset, const CSSStyleSheetResource*);