Fix Win compilation issue introduced in r8081.

Review URL: http://codereview.chromium.org/7074009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8086 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/runtime.cc

@@ -5109,7 +5109,7 @@ static inline SinkChar* WriteQuoteJsonString(
     SourceChar c = *(read_cursor++);
     if (sizeof(SourceChar) > 1u &&
         static_cast<unsigned>(c) >= kQuoteTableLength) {
-      *(write_cursor++) = c;
+      *(write_cursor++) = static_cast<SinkChar>(c);
     } else {
       int len = JsonQuoteLengths[static_cast<unsigned>(c)];
       const char* replacement = JsonQuotes +