Also create new v8 versions based on the roll ref, which
has some more verification steps than the candidate ref.

Improve bailout regarding existing versions. Only create a
new version if the revisions is in the future.

BUG=chromium:451975
TBR=tandrii@chromium.org
NOTRY=true
LOG=n

Review URL: https://codereview.chromium.org/878913002

Cr-Commit-Position: refs/heads/master@{#26303}