[d8] Fix V8 external snapshot for Windows.

src/d8.cc

Index: src/d8.cc
diff --git a/src/d8.cc b/src/d8.cc
index 132891e2b8a694919c0643118dea5fd8ef82b403..7157cb81acf85171b94e331ad02e8404323dd0c5 100644
--- a/src/d8.cc
+++ b/src/d8.cc
@@ -1577,7 +1577,7 @@ class StartupDataHandler {
     const char* last_slash = strrchr(exec_path, '/');
     if (last_slash) {
       int after_slash = last_slash - exec_path + 1;
-      int name_length = strlen(name);
+      int name_length = static_cast<int>(strlen(name));
       *buffer =
           reinterpret_cast<char*>(calloc(after_slash + name_length + 1, 1));
       strncpy(*buffer, exec_path, after_slash);