Make atob() / btoa() argument non optional

According to the latest specification, the argument to atob() / btoa()
should not be optional:
http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#windowbase64

This patch makes Blink behave according to the specification. The argument
is also mandatory in Firefox and IE10.

atob() / btoa() are also moved to their own WindowBase64 interface which
the Window interface implements. This does not change the behavior but
this is closer to the specification and it will make exposing those
methods to workers easier later on.

BUG=261126

Review URL: https://chromiumcodereview.appspot.com/19576002

git-svn-id: svn://svn.chromium.org/blink/trunk@154419 bbb929c8-8fbe-4397-9dbb-9b2b20218538