Minor refactoring of code in TextIterator

This patch avoid unneccessary function calls, by using the already avaiable member variable.
No new test cases are needed

Change-Id: If7ac8eecc4e8fbfa0a4887deec4bc97a4826da48

BUG=NONE

Review URL: https://codereview.chromium.org/107103005

git-svn-id: svn://svn.chromium.org/blink/trunk@163994 bbb929c8-8fbe-4397-9dbb-9b2b20218538