Add Stream support to FileReaderLoader.

To be used directly by Stream (with Promises) or FileReader.

BUG=169957

Review URL: https://chromiumcodereview.appspot.com/18991017

git-svn-id: svn://svn.chromium.org/blink/trunk@154111 bbb929c8-8fbe-4397-9dbb-9b2b20218538