Refactoring: Rename 'Alias' to 'PastNames'.

'Past names' is a term used in the standard. [1]
This imports a part of WebKit r154586 by rniwa. [2]

[1] http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#dom-form-nameditem
[2] http://trac.webkit.org/changeset/154586

BUG=190026
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23726008

git-svn-id: svn://svn.chromium.org/blink/trunk@156975 bbb929c8-8fbe-4397-9dbb-9b2b20218538