Remove mutable keyword from member variables of XMLHttpRequest.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@160682 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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