Add "description" attribute to Value.

tools/telemetry/telemetry/value/scalar.py

Index: tools/telemetry/telemetry/value/scalar.py
diff --git a/tools/telemetry/telemetry/value/scalar.py b/tools/telemetry/telemetry/value/scalar.py
index 026f425b41646d88a72da59abeb1456bd700b330..dbb3a410496f48bbc19af592e7cd622109bddb51 100644
--- a/tools/telemetry/telemetry/value/scalar.py
+++ b/tools/telemetry/telemetry/value/scalar.py
@@ -8,14 +8,15 @@ from telemetry import value as value_module
 from telemetry.value import list_of_scalar_values
 
 class ScalarValue(value_module.Value):
-  def __init__(self, page, name, units, value, important=True):
+  def __init__(self, page, name, units, value, important=True,
+               description=None):
     """A single value (float or integer) result from a test.
 
     A test that counts the number of DOM elements in a page might produce a
     scalar value:
        ScalarValue(page, 'num_dom_elements', 'count', num_elements)
     """
-    super(ScalarValue, self).__init__(page, name, units, important)
+    super(ScalarValue, self).__init__(page, name, units, important, description)
     assert isinstance(value, numbers.Number)
     self.value = value
 
@@ -24,11 +25,13 @@ class ScalarValue(value_module.Value):
       page_name = self.page.url
     else:
       page_name = None
-    return 'ScalarValue(%s, %s, %s, %s, important=%s)' % (
+    return 'ScalarValue(%s, %s, %s, %s, important=%s, description=%s)' % (
       page_name,
-      self.name, self.units,
+      self.name,
+      self.units,
       self.value,
-      self.important)
+      self.important,
+      self.description)
 
   def GetBuildbotDataType(self, output_context):
     if self._IsImportantGivenOutputIntent(output_context):