This allows variants to be named on test failures (follow
up) and then to be used in the test runner for a repro.

This also speeds up variant iteration for test262 and fixes
a bug with variants for benchmarks.

BUG=chromium:511215
NOTREECHECKS=true
LOG=n

Review URL: https://codereview.chromium.org/1245623005

Cr-Commit-Position: refs/heads/master@{#29899}