Fix style error in core/editing/TextIterator.cpp

This patch fixes style error in core/editing/TextIterator.cpp for introducing PlainTextRange. PlainTextRange.cpp will be copied from TextIterator.cpp for moving getLocationAndLengthFromRange() and rangeFromLocationAndLength() to PlainTextRange class.

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

Review URL: https://codereview.chromium.org/34523010

git-svn-id: svn://svn.chromium.org/blink/trunk@160312 bbb929c8-8fbe-4397-9dbb-9b2b20218538