Move StyleSheet to the oilpan heap using transition types.

Source/core/css/MediaList.cpp

Index: Source/core/css/MediaList.cpp
diff --git a/Source/core/css/MediaList.cpp b/Source/core/css/MediaList.cpp
index d0698cef119c8a313756f548010ba4af84ba036b..28232dacb63bcad20d37a75ca26e87e667180f51 100644
--- a/Source/core/css/MediaList.cpp
+++ b/Source/core/css/MediaList.cpp
@@ -176,7 +176,7 @@ MediaList::MediaList(MediaQuerySet* mediaQueries, CSSStyleSheet* parentSheet)
 
 MediaList::MediaList(MediaQuerySet* mediaQueries, CSSRule* parentRule)
     : m_mediaQueries(mediaQueries)
-    , m_parentStyleSheet(0)
+    , m_parentStyleSheet(nullptr)
     , m_parentRule(parentRule)
 {
 }
@@ -239,6 +239,7 @@ void MediaList::reattach(MediaQuerySet* mediaQueries)
 void MediaList::trace(Visitor* visitor)
 {
     visitor->trace(m_mediaQueries);
+    visitor->trace(m_parentStyleSheet);
     visitor->trace(m_parentRule);
 }