Reduce TextIterator dependency from accessibility module

This patch reduces TextIterator dependency from accessibility module. The usage of TextIterator is only in AXRenderObject.

BUG=n/a
TEST=n/a; no behavior changes
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/35773003

git-svn-id: svn://svn.chromium.org/blink/trunk@160305 bbb929c8-8fbe-4397-9dbb-9b2b20218538