Separates PreviousSessionInfo testing interface

This CL moves PreviousSessionInfo testing interface in a different header file.
It also exposes isFirstSessionAfterUpgrade property as read-write for testing.

BUG=none

Review-Url: https://codereview.chromium.org/1984223002
Cr-Commit-Position: refs/heads/master@{#396172}
6 files changed