[oilpan] Fix iterators over member collections to return Result<T>

Source/heap/Visitor.h

Index: Source/heap/Visitor.h
diff --git a/Source/heap/Visitor.h b/Source/heap/Visitor.h
index 06c4c1ac5fb2e652479cdfeeceafaf5c46e3cc56..47aecf6ec866301a99bed42ba7cdc0aa3f524db3 100644
--- a/Source/heap/Visitor.h
+++ b/Source/heap/Visitor.h
@@ -39,6 +39,7 @@
 namespace WebCore {
 
 template<typename T> class Member;
+template<typename T> class Result;
 
 #ifndef NDEBUG
 #define DECLARE_GC_TYPE_MARKER         \
@@ -118,6 +119,21 @@ public:
         AcceptTrait<T>::visit(this, t.raw());
     }
 
+    // Result version of the one-argument templated visit method. This is used
+    // for visiting collections: The iterator returns Result<T> and  we visit
+    // them. This visitor method is befriended by Result<T> which allows it to
+    // use releaseRaw.
+    template<typename T>
+    void visit(const Result<T>& t)
+    {
+        if (!t)
+            return;
+#ifndef NDEBUG
+        AcceptTrait<T>::check(this, t.releaseRaw());
+#endif
+        AcceptTrait<T>::visit(this, t.releaseRaw());
+    }
+
     template<typename T, size_t N>
     void visit(const Vector<Member<T>, N>& vector)
     {