Fix cast that fails on Win64.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 58cde3b10a215a230565f776f36fc1a9eab0b784..af40e983b9a480e4d0ce6993764b1c6ef7888fc1 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -4723,9 +4723,9 @@ static MaybeObject* QuoteJsonString(Vector<const Char> characters) {
   }
   *(write_cursor++) = '"';
 
-  int final_length =
+  int final_length = static_cast<int>(
       write_cursor - reinterpret_cast<Char*>(
-          new_string->address() + SeqAsciiString::kHeaderSize);
+          new_string->address() + SeqAsciiString::kHeaderSize));
   Heap::new_space()->ShrinkStringAtAllocationBoundary<StringType>(new_string,
                                                                   final_length);
   return new_string;