[heap] Remove store buffer top from roots

src/heap/store-buffer.h

Index: src/heap/store-buffer.h
diff --git a/src/heap/store-buffer.h b/src/heap/store-buffer.h
index 06a99a8dcc8e04673172cc8fe5565eb9c3ee3400..9c6eac90b5897bb02fe84bb6e360880c4c42860d 100644
--- a/src/heap/store-buffer.h
+++ b/src/heap/store-buffer.h
@@ -18,20 +18,26 @@ namespace internal {
 // code. On buffer overflow the slots are moved to the remembered set.
 class StoreBuffer {
  public:
-  explicit StoreBuffer(Heap* heap);
+  static const int kStoreBufferOverflowBit = 1 << (14 + kPointerSizeLog2);
+  static const int kStoreBufferSize = kStoreBufferOverflowBit;
+  static const int kStoreBufferLength = kStoreBufferSize / sizeof(Address);
+
   static void StoreBufferOverflow(Isolate* isolate);
+
+  explicit StoreBuffer(Heap* heap);
   void SetUp();
   void TearDown();
 
-  static const int kStoreBufferOverflowBit = 1 << (14 + kPointerSizeLog2);
-  static const int kStoreBufferSize = kStoreBufferOverflowBit;
-  static const int kStoreBufferLength = kStoreBufferSize / sizeof(Address);
+  // Used to add entries from generated code.
+  inline Address* top_address() { return reinterpret_cast<Address*>(&top_); }
 
   void MoveEntriesToRememberedSet();
 
  private:
   Heap* heap_;
 
+  Address* top_;
+
   // The start and the limit of the buffer that contains store slots
   // added from the generated code.
   Address* start_;