[heap] Remove store buffer top from roots

src/heap/store-buffer.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_STORE_BUFFER_H_
 #define V8_STORE_BUFFER_H_
 
 #include "src/allocation.h"
 #include "src/base/logging.h"
 #include "src/base/platform/platform.h"
 #include "src/globals.h"
 #include "src/heap/slot-set.h"
 
 namespace v8 {
 namespace internal {
 
 // Intermediate buffer that accumulates old-to-new stores from the generated
 // code. On buffer overflow the slots are moved to the remembered set.
 class StoreBuffer {
  public:
+  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);
-  static void StoreBufferOverflow(Isolate* isolate);
   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_;
   Address* limit_;
 
   base::VirtualMemory* virtual_memory_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_STORE_BUFFER_H_