Make auto-roller roll recent release based on timestamp.

Make auto-roller roll recent release based on timestamp.

Before this change, out of two versions 4.2.13.1 and
4.2.14, the latter would have been considered the newest.
Now, the timestamp of the commit determines the age, which
allows to roll a patched version first. The auto-roller
will only roll forward (i.e. there needs to be a commit
range between the last roll and the new candidate).

Additionally, this CL does some cleanups:
- Use the sheriff detection mechanism in the auto-roller.
- Require the roll revision parameter in the chromium_roll
script to avoid redundancy. The auto_roll script determines
that revision automatically.
- Simplify the revision summary in the commit message. The
summary will now show last_roll..new_roll, which e.g.
includes the version change CL. It'll now show useful
information for cherry-picks, which it didn't before.
- Remove unused clusterfuzz check. That check is part of the
release process script.

TBR=tandrii@chromium.org
NOTRY=true
TEST=./script_test.py
TEST=./tools/release/chromium_roll.py --dry-run --sheriff -c ~/tmp/chromium/src --last-roll 55b9049ea3978a589d8db2aa191d21222eef737f fc263505535a175c8efa18f4c787bd92a1d1ab3d --use-commit-queue -r me -a you

Review URL: https://codereview.chromium.org/977903002

Cr-Commit-Position: refs/heads/master@{#27009}