Use tighter typing in editing: CompositeEditCommand

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

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/424493003

git-svn-id: svn://svn.chromium.org/blink/trunk@178986 bbb929c8-8fbe-4397-9dbb-9b2b20218538