Make Stream implement ActiveDOMObject.

To implement the read() method of Stream, we'll use FileReaderLoader that
involves asynchronous operation.

BUG=169957

Review URL: https://codereview.chromium.org/25830006

git-svn-id: svn://svn.chromium.org/blink/trunk@159746 bbb929c8-8fbe-4397-9dbb-9b2b20218538