JSONParser: properly decode escaped values (\uXXXX).

BUG=304131
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/26184005

git-svn-id: svn://svn.chromium.org/blink/trunk@159089 bbb929c8-8fbe-4397-9dbb-9b2b20218538