New Guest mode avatar on the start screen

BUG=140417
TEST=units, out/Debug/chrome --disable-seccomp-sandbox --parallel-auth --login-manager --login-profile=user --log-level=0 --enable-logging=stderr --no-first-run --user-data-dir=/tmp/user --in-chrome-auth --scroll-pixels=3 --enable-smooth-scrolling --force-compositing-mode --force-device-scale-factor=2 --load-2x-resources

Review URL: https://chromiumcodereview.appspot.com/10829166

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@150937 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/resources/chromeos/login/user_pod_row.js

@@ -324,7 +324,7 @@ cr.define('login', function() {
      */
     updateUserImage: function() {
       this.imageElement.src = this.isGuest ?
-          'chrome://theme/IDR_LOGIN_GUEST' :
+          'chrome://theme/IDR_LOGIN_GUEST@' + window.devicePixelRatio + 'x' :
           'chrome://userimage/' + this.user.username +
               '?id=' + new Date().getTime();
     },