Improve C++11 in /fetch
    
Use the actual type in places where it makes the code clearer and not
any longer. Fix spelling error. Use nullptr.

Review URL: https://codereview.chromium.org/665613002

git-svn-id: svn://svn.chromium.org/blink/trunk@183964 bbb929c8-8fbe-4397-9dbb-9b2b20218538