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..7e2759a497e7a25f281113bbc5346a0437b505c0
--- /dev/null
+++ b/tools/telemetry/telemetry/value/none_values.py
@@ -0,0 +1,24 @@
+# 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 ValidateNoneValueReason(value, none_value_reason):
+  """Ensures that the none_value_reason is appropriate for the given value.
+
+  There is a logical equality between having a value of None and having a
+  reason for being None. That is to say, value is None if and only if
+  none_value_reason is a string.
+  """
+  if value is None and not isinstance(none_value_reason, basestring):
+    raise NoneValueMissingReason()
+  if value is not None and none_value_reason is not None:
+    raise ValueMustHaveNoneValue()