Fix warning from Windows about loss of data in cast-less assignment.

Review URL: http://codereview.chromium.org/7865

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@552 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/utils.h

@@ -448,7 +448,7 @@ class StringBuilder {
 template <typename sourcechar, typename sinkchar>
 static inline void CopyChars(sinkchar* dest, const sourcechar* src, int chars) {
   while (chars--) {
-    *dest++ = *src++;
+    *dest++ = static_cast<sinkchar>(*src++);
   }
 }