Fix bug in Value.getBaseline().

The method wasn't being overridden by InstanceValue.getBaseline as
intended, which would lead to broken links in the UI.

Test: m ahat-test, with regression test added.
Test: Manually confirm a link to a "was" part of a diffed field works
      in the UI.
Bug: 77943151

Change-Id: I98a40afe2161617ba2199574b4fd57a076fc8bde
2 files changed