Add "description" attribute to Value.

tools/telemetry/telemetry/value/histogram.py

Index: tools/telemetry/telemetry/value/histogram.py
diff --git a/tools/telemetry/telemetry/value/histogram.py b/tools/telemetry/telemetry/value/histogram.py
index d57f6f57c523980992e9351d94e83d0b1ce5dd3b..fa70445c7232f70c936af0d14773401521128c0e 100644
--- a/tools/telemetry/telemetry/value/histogram.py
+++ b/tools/telemetry/telemetry/value/histogram.py
@@ -27,8 +27,10 @@ class HistogramValueBucket(object):
 
 class HistogramValue(value_module.Value):
   def __init__(self, page, name, units,
-               raw_value=None, raw_value_json=None, important=True):
-    super(HistogramValue, self).__init__(page, name, units, important)
+               raw_value=None, raw_value_json=None, important=True,
+               description=None):
+    super(HistogramValue, self).__init__(page, name, units, important,
+                                         description)
     if raw_value_json:
       assert raw_value == None, 'Dont specify both raw_value and raw_value_json'
       raw_value = json.loads(raw_value_json)
@@ -49,11 +51,13 @@ class HistogramValue(value_module.Value):
       page_name = self.page.url
     else:
       page_name = None
-    return 'HistogramValue(%s, %s, %s, raw_json_string="%s", important=%s)' % (
-      page_name,
-      self.name, self.units,
-      self.ToJSONString(),
-      self.important)
+    return ('HistogramValue(%s, %s, %s, raw_json_string="%s", '
+            'important=%s, description=%s') % (
+              page_name,
+              self.name, self.units,
+              self.ToJSONString(),
+              self.important,
+              self.description)
 
   def GetBuildbotDataType(self, output_context):
     if self._IsImportantGivenOutputIntent(output_context):