Have Text constructor take a Document reference in argument

Have Text constructor take a Document reference in argument as it can
never be NULL. Most callers already have a reference so this avoids
having to convert them into pointers.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23513013

git-svn-id: svn://svn.chromium.org/blink/trunk@157251 bbb929c8-8fbe-4397-9dbb-9b2b20218538