Make DataTransferItem.getAsString() argument mandatory and nullable

Make DataTransferItem.getAsString() argument mandatory and nullable to
match the latest specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/dnd.html#dom-datatransferitem-getasstring

BUG=269548
R=arv@chromium.org, haraken@chromium.org, tkent@chromium.org

Review URL: https://codereview.chromium.org/22604003

git-svn-id: svn://svn.chromium.org/blink/trunk@155747 bbb929c8-8fbe-4397-9dbb-9b2b20218538