[RuntimeStats] Show all entries which are in at least one version.

Previously the runtime stats tool would only show entries which were in
the baseline version. This change adds any entries which exist in any
version to the comparison table, which would otherwise not appear anywhere
unless that version was selected as the baseline version.

NOTRY=true

Review-Url: https://codereview.chromium.org/2683863004
Cr-Commit-Position: refs/heads/master@{#43045}