First step in moving all statusfile logic into statusfile.py.

Introduce StatusFile object that will be used for storing and managing
outcomes.

Bug: v8:6917
Change-Id: I024f9b1d029830345149422a08a8905e92545252
Reviewed-on: https://chromium-review.googlesource.com/832433
Commit-Queue: Michał Majewski <majeski@google.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50202}