Make Win64 compile.

src/spaces.h

Index: src/spaces.h
diff --git a/src/spaces.h b/src/spaces.h
index e71796084024e5f67cc7641e008fcda742be01db..ec8b9d9d7806b1eec0998916ddfb0cff0f3a13d5 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -352,7 +352,7 @@ class MemoryChunk {
 
   Address body_limit() { return address() + size(); }
 
-  int body_size() { return size() - kObjectStartOffset; }
+  int body_size() { return static_cast<int>(size() - kObjectStartOffset); }
 
   bool Contains(Address addr) {
     return addr >= body() && addr < address() + size();