Remove support for objects w/ indexed properties in the Blob/File constructors.

The Blob and File constructor implementations in Blink support the
WebIDL sequence interpretation in the W3C Editor's Draft on 9 July 2014.
However, Firefox implements a different interpretation, based on the "for
... of" ES6 construct.

This change resticts the Blob and File constructor support to JavaScript
arrays, which are the most common used type of WebIDL sequences. The
change removes support for objects with indexed properties, to avoid
having Web developers rely on this functionality.

BUG=

Review URL: https://codereview.chromium.org/391813005

git-svn-id: svn://svn.chromium.org/blink/trunk@178171 bbb929c8-8fbe-4397-9dbb-9b2b20218538