Have ContentData::createRenderer() take a Document reference in argument

Have ContentData::createRenderer() take a Document reference in argument
instead of a pointer as it can never be NULL. It also avoids having to
convert a reference into a pointer at call sites.

R=tkent
BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23731003

git-svn-id: svn://svn.chromium.org/blink/trunk@157084 bbb929c8-8fbe-4397-9dbb-9b2b20218538