Use tighter typing in editing: markup

Use tighter typing in editing to increase code readability and avoid
unnecessary type checks. This CL focuses on markup.

Review URL: https://codereview.chromium.org/428823002

git-svn-id: svn://svn.chromium.org/blink/trunk@179169 bbb929c8-8fbe-4397-9dbb-9b2b20218538