Have EditCommand classes deal with Document references, not pointers

Have EditCommand classes deal with Document references, not pointers, as it
can never be NULL.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23822003

git-svn-id: svn://svn.chromium.org/blink/trunk@157085 bbb929c8-8fbe-4397-9dbb-9b2b20218538