Use tighter typing in editing: Editor / EditorCommand

Use tighter typing in editing to increase code readability. This CL focuses on
Editor and EditorCommand.

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/426433002

git-svn-id: svn://svn.chromium.org/blink/trunk@178988 bbb929c8-8fbe-4397-9dbb-9b2b20218538