Have Attr constructor take a Document / Element reference

Have Attr constructor take a Document / Element reference instead of a pointer
as it can never be NULL.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23567024

git-svn-id: svn://svn.chromium.org/blink/trunk@157481 bbb929c8-8fbe-4397-9dbb-9b2b20218538