Expose FileList in Worker contexts.

The File API spec exposes FileList in both Window and Worker contexts,

 http://dev.w3.org/2006/webapi/FileAPI/#filelist-section

Follow up and have the implementation also provide the latter.

R=michaeln@chromium.org,jsbell@chromium.org
BUG=339441

Review URL: https://codereview.chromium.org/349633005

git-svn-id: svn://svn.chromium.org/blink/trunk@177456 bbb929c8-8fbe-4397-9dbb-9b2b20218538