Add a sub_test_name parameter to the mojo LogPerfResult().

When the perf data is used by the performance dashboard, data with
different |sub_test_name|s (but the same |test_name|) are depicted as
different traces on the same chart.

BUG=None
TEST=None
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/804683002