Remove underscores from mode identifiers.

Underscore is used as a field separator sometimes when parsing the task
name into a list of config, mode, etc.  (This itself is dumb and TODO(mtklein): fix.)
Underscores in the field names will really mess that up, both in directories generated
from human-mode -w, and in the .json file.

BUG=skia:
R=jcgregorio@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/599503002
2 files changed