Oilpan: restrict implicit RawPtr<U> conversions; U must be T-convertible.

Source/wtf/RawPtr.h

Index: Source/wtf/RawPtr.h
diff --git a/Source/wtf/RawPtr.h b/Source/wtf/RawPtr.h
index 3175c1e2ffa9debb97c46334b7dc1556763327a9..758bb12c14d711e34251e5366862a1f31ff9b672 100644
--- a/Source/wtf/RawPtr.h
+++ b/Source/wtf/RawPtr.h
@@ -35,6 +35,7 @@
 #include <stdint.h>
 
 #include "wtf/HashTableDeletedValueType.h"
+#include "wtf/TypeTraits.h"
 
 // RawPtr is a simple wrapper for a raw pointer that provides the
 // interface (get, clear) of other pointer types such as RefPtr,
@@ -63,7 +64,7 @@ public:
     }
 
     template<typename U>
-    RawPtr(const RawPtr<U>& other)
+    RawPtr(const RawPtr<U>& other, EnsurePtrConvertibleArgDecl(U, T))
         : m_ptr(other.get())
     {
     }