Remove debug-only code which is no longer meaningful.

TextIterator's constructor initializes m_positionNode, but subsequent advance()
resets the variable to null, so there is no point in initializing it in
the constructor.

BUG=none
R=yosin@chromium.org

Review URL: https://codereview.chromium.org/89703004

git-svn-id: svn://svn.chromium.org/blink/trunk@162744 bbb929c8-8fbe-4397-9dbb-9b2b20218538