Support None value for telemetry values.

tools/telemetry/telemetry/value/none_values.py

Index: tools/telemetry/telemetry/value/none_values.py
diff --git a/tools/telemetry/telemetry/value/none_values.py b/tools/telemetry/telemetry/value/none_values.py
new file mode 100644
index 0000000000000000000000000000000000000000..8738c764b35ad200014ad28768fe84fbc2c62480
--- /dev/null
+++ b/tools/telemetry/telemetry/value/none_values.py
@@ -0,0 +1,18 @@
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+MERGE_FAILURE_REASON = (
+    'Merging values containing a None value results in a None value')
+
+class NoneValueMissingReason(Exception):
+  pass
+
+class ValueMustHaveNoneValue(Exception):
+  pass
+
+def ValidateNullableValue(value, none_value_reason):

[nednguyen, 2014/08/18 06:54:57]

Validate none_value_reason's type since the AsDict method can apply for it?

[ariblue, 2014/08/18 17:56:45]

Done.

+  if value is None and none_value_reason is None:
+    raise NoneValueMissingReason()
+  if value is not None and none_value_reason is not None:
+    raise ValueMustHaveNoneValue()