mojo_benchmark: don't duplicate benchmark name in variant name.

mojo_benchmark: don't duplicate benchmark name in variant name.

Thich patch separates benchmark_name (original name of the benchmark
from the spec file) from variant_name (just the name of the variant,
e.g. "cold start"). This will allow to record the results hierarchically
without duplication (ie. as "benchmark_name/variant_name").

R=etiennej@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/76400e028dee21ae506cf016f52ad2dc594defad

[ppi, 2015-10-26 14:25:47]

ppi@chromium.org changed reviewers:
+ etiennej@chromium.org

[ppi, 2015-10-26 14:25:47]

Hi Etienne,

ptal.

[etiennej, 2015-10-27 10:37:32]

lgtm

[ppi, 2015-10-27 10:56:08]

Committed patchset #2 (id:20001) manually as
76400e028dee21ae506cf016f52ad2dc594defad (presubmit successful).