Add ES specification reference to DateNow.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/89353002

Patch from Victor Costan <costan@gmail.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18094 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/date.js

@@ -302,8 +302,7 @@ function DateUTC(year, month, date, hours, minutes, seconds, ms) {
 }
 
 
-// Mozilla-specific extension. Returns the number of milliseconds
-// elapsed since 1 January 1970 00:00:00 UTC.
+// ECMA 262 - 15.9.4.4
 function DateNow() {
   return %DateCurrentTime();
 }