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
diff --git a/dm/DMSerializeTask.cpp b/dm/DMSerializeTask.cpp
index 8116a2e..161836b 100644
--- a/dm/DMSerializeTask.cpp
+++ b/dm/DMSerializeTask.cpp
@@ -9,7 +9,7 @@
 DEFINE_bool(serialize,     true, "If true, run picture serialization tests via SkPictureData.");
 DEFINE_bool(serialize_skr, true, "If true, run picture serialization tests via SkRecord.");
 
-static const char* kSuffixes[] = { "serialize", "serialize_skr" };
+static const char* kSuffixes[] = { "serialize", "serialize-skr" };
 static const bool* kEnabled[]  = { &FLAGS_serialize, &FLAGS_serialize_skr };
 
 namespace DM {