Use tighter typing in editing: PlainTextRange / ReplaceNodeWithSpanCommand

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

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

Review URL: https://codereview.chromium.org/430843002

git-svn-id: svn://svn.chromium.org/blink/trunk@179318 bbb929c8-8fbe-4397-9dbb-9b2b20218538