Let release scripts determine version based on tags.

Let release scripts determine version based on tags.

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}