Have CustomElement::didEnterDocument() / didLeaveDocument() take a reference

Have CustomElement::didEnterDocument() / didLeaveDocument() take a Document
reference instead of a pointer since it cannot be NULL. It also avoids
having the call sites convert the Document reference into a pointer.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23523012

git-svn-id: svn://svn.chromium.org/blink/trunk@157043 bbb929c8-8fbe-4397-9dbb-9b2b20218538