Fix style errors in core/editing/Editor.cpp

This patch fixes style errors in core/editing/Editor.cpp for preparation of decoupling spell checker and Editor class.

Another patch will add SpellChecker.cpp and SpellChecker.h as copy of Editor.cpp and Editor.h.

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

Review URL: https://codereview.chromium.org/26627005

git-svn-id: svn://svn.chromium.org/blink/trunk@159302 bbb929c8-8fbe-4397-9dbb-9b2b20218538