[heap] Move IdentityMap data structure out of heap.

src/identity-map.h

 // Copyright 2015 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_HEAP_IDENTITY_MAP_H_
-#define V8_HEAP_IDENTITY_MAP_H_
+#ifndef V8_IDENTITY_MAP_H_
+#define V8_IDENTITY_MAP_H_
 
 #include "src/handles.h"
 
 namespace v8 {
 namespace internal {
 
 // Forward declarations.
 class Heap;
 class Zone;
 
@@ skipping 71 matching lines @@
   V* Find(Handle<Object> key) { return reinterpret_cast<V*>(FindEntry(key)); }
 
   // Set the value for the given key.
   void Set(Handle<Object> key, V value) {
     *(reinterpret_cast<V*>(GetEntry(key))) = value;
   }
 };
 }
 }  // namespace v8::internal
 
-#endif  // V8_HEAP_IDENTITY_MAP_H_
+#endif  // V8_IDENTITY_MAP_H_