Use tighter typing in editing/

Use tighter typing in editing/ to improve code readability. This CL focuses
mainly on htmlediting.cpp / htmlediting.h.

R=adamk@chromium.org, leviw@chromium.org

Review URL: https://codereview.chromium.org/420603002

git-svn-id: svn://svn.chromium.org/blink/trunk@178881 bbb929c8-8fbe-4397-9dbb-9b2b20218538