Add "description" attribute to Value.

tools/telemetry/telemetry/value/list_of_string_values.py

Index: tools/telemetry/telemetry/value/list_of_string_values.py
diff --git a/tools/telemetry/telemetry/value/list_of_string_values.py b/tools/telemetry/telemetry/value/list_of_string_values.py
index f029279a316dc085b25e628a1c068ee5eaa4d86f..9e8566a80c2c5cf4c778afef7da30bf839e6e3a3 100644
--- a/tools/telemetry/telemetry/value/list_of_string_values.py
+++ b/tools/telemetry/telemetry/value/list_of_string_values.py
@@ -6,8 +6,10 @@ from telemetry import value as value_module
 
 class ListOfStringValues(value_module.Value):
   def __init__(self, page, name, units, values,
-               important=True, same_page_merge_policy=value_module.CONCATENATE):
-    super(ListOfStringValues, self).__init__(page, name, units, important)
+               important=True, description=None,
+               same_page_merge_policy=value_module.CONCATENATE):
+    super(ListOfStringValues, self).__init__(page, name, units, important,
+                                             description)
     assert len(values) > 0
     assert isinstance(values, list)
     for v in values:
@@ -25,11 +27,13 @@ class ListOfStringValues(value_module.Value):
     else:
       merge_policy = 'PICK_FIRST'
     return ('ListOfStringValues(%s, %s, %s, %s, ' +
-            'important=%s, same_page_merge_policy=%s)') % (
+            'important=%s, description=%s, same_page_merge_policy=%s)') % (
               page_name,
-              self.name, self.units,
+              self.name,
+              self.units,
               repr(self.values),
               self.important,
+              self.description,
               merge_policy)
 
   def GetBuildbotDataType(self, output_context):