Remove underscores from mode identifiers.

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:

Committed: https://skia.googlesource.com/skia/+/e51ac563de24ed4b25fde97c225580a30a55e9ca

[mtklein, 2014-09-23 15:29:56]

mtklein@google.com changed reviewers:
+ jcgregorio@google.com, mtklein@google.com

[jcgregorio, 2014-09-23 15:32:22]

On 2014/09/23 15:29:57, mtklein wrote:

LGTM

[mtklein, 2014-09-23 15:33:42]

The CQ bit was checked by mtklein@google.com

[mtklein, 2014-09-23 15:33:43]

lgtm

[mtklein, 2014-09-23 15:33:57]

On 2014/09/23 15:33:43, mtklein wrote:
> lgtm

Oops.  Well, it does.

[commit-bot: I haz the power, 2014-09-23 15:34:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/599503002/1

[commit-bot: I haz the power, 2014-09-23 16:20:18]

Committed patchset #1 (id:1) as e51ac563de24ed4b25fde97c225580a30a55e9ca