Expose atob() / btoa() to workers

Expose atob() / btoa() to workers as per the latest Web Workers
specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/workers.html#WorkerGlobalScope-partial
http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#windowbase64

This behavior is consistent with Firefox and IE10.

BUG=143502
R=arv@chromium.org, haraken@chromium.org

Review URL: https://codereview.chromium.org/19749003

git-svn-id: svn://svn.chromium.org/blink/trunk@154578 bbb929c8-8fbe-4397-9dbb-9b2b20218538