Oilpan: fix build after r201639.

TBR=oilpan-reviews
BUG=none
NOTRY=true

Review URL: https://codereview.chromium.org/1312073003

git-svn-id: svn://svn.chromium.org/blink/trunk@201647 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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

@@ -58,6 +58,7 @@ public:
     const CSSStyleSheet* parentStyleSheet() const { return m_parentStyleSheet; }
     DEFINE_INLINE_TRACE()
     {
+        visitor->trace(m_ruleData);
         visitor->trace(m_parentStyleSheet);
     }