Remove mutable keyword from member variables of XMLHttpRequest.

This CL removes mutable keyword as these are no longer used in const functions.

Review URL: https://codereview.chromium.org/43653003

git-svn-id: svn://svn.chromium.org/blink/trunk@160682 bbb929c8-8fbe-4397-9dbb-9b2b20218538