This introduces aliases for testing variants that can be
combined with other variant names. E.g. --variants=dev,foo
would run the three developer default variants and variant
foo.

We'll have three stages: "dev" for variants to be run by
default on developer workstations, "more" for additional
variants, executed on all bots, "extra" for additional
variants executed on a subset of bots (e.g. not on very slow
or otherwise resource-limited bots).

BUG=v8:5238
NOTRY=true

Review-Url: https://codereview.chromium.org/2196223002
Cr-Commit-Position: refs/heads/master@{#38214}