Subzero: Switch file reading to be based on a DataStreamer and MemoryObject.

This makes it compatible with the current browser interface,
which pushes bytes to a DataStreamer.

In the browser-integration mode, there will be a push-based
DataStreamer (vs a file-based one).

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4091
R=dschuff@chromium.org, kschimpf@google.com, stichnot@chromium.org

Review URL: https://codereview.chromium.org/982403002