Perf: Fix description for trace_cov.
Perf: Fix description for trace_cov.

BUG=skia:

Change-Id: Iaadfcb70168ebfd7d39e975b7f1c09b7bd8a77d3
Reviewed-on: https://skia-review.googlesource.com/9969
Reviewed-by: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@chromium.org>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot