Oilpan: Clarify ownership of split-off heaps.

Source/platform/heap/Heap.h

Index: Source/platform/heap/Heap.h
diff --git a/Source/platform/heap/Heap.h b/Source/platform/heap/Heap.h
index b1d94dd7f66445ffd2c492f39f9f905fdae7e172..1d62554c703b5336932329cfa2ecc7439e6332f8 100644
--- a/Source/platform/heap/Heap.h
+++ b/Source/platform/heap/Heap.h
@@ -722,8 +722,8 @@ public:
 
     virtual int normalPageCount() = 0;
 
-    virtual BaseHeap* split(int normalPages) = 0;
-    virtual void merge(BaseHeap* other) = 0;
+    virtual PassOwnPtr<BaseHeap> split(int normalPages) = 0;
+    virtual void merge(PassOwnPtr<BaseHeap> other) = 0;
 
     // Returns a bucket number for inserting a FreeListEntry of a
     // given size. All FreeListEntries in the given bucket, n, have
@@ -784,8 +784,8 @@ public:
 
     virtual int normalPageCount() { return m_numberOfNormalPages; }
 
-    virtual BaseHeap* split(int numberOfNormalPages);
-    virtual void merge(BaseHeap* splitOffBase);
+    virtual PassOwnPtr<BaseHeap> split(int numberOfNormalPages) override;
+    virtual void merge(PassOwnPtr<BaseHeap> splitOffBase) override;
 
     void removePageFromHeap(HeapPage<Header>*);