Remove unnecessary mutable keyword for m_nodeFlags

This CL deletes unneeded mutable keyword by removing const keyword on setter functions.

Review URL: https://codereview.chromium.org/337113002

git-svn-id: svn://svn.chromium.org/blink/trunk@176210 bbb929c8-8fbe-4397-9dbb-9b2b20218538