Use tighter typing in editing: MarkupAccumulator

Use tighter typing in editing/ to increase code readability. This CL focuses on
MarkupAccumulator.

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

Review URL: https://codereview.chromium.org/426293002

git-svn-id: svn://svn.chromium.org/blink/trunk@179488 bbb929c8-8fbe-4397-9dbb-9b2b20218538