Fix build problems on Windows 64-bit by casting.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 4aaf2d563926352b68ecbadcc82b06bcc1fe0c75..b4977548330d95c2c7dd31099d47f210b5338a4d 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -2307,7 +2307,8 @@ Object* Heap::CopyCode(Code* code, Vector<byte> reloc_info) {
 
   Address old_addr = code->address();
 
-  int relocation_offset = code->relocation_start() - old_addr;
+  size_t relocation_offset =
+      static_cast<size_t>(code->relocation_start() - old_addr);
 
   Object* result;
   if (new_obj_size > MaxObjectSizeInPagedSpace()) {