[Telemetry] Add capability for values to reference external files.

tools/telemetry/telemetry/value/__init__.py

Index: tools/telemetry/telemetry/value/__init__.py
diff --git a/tools/telemetry/telemetry/value/__init__.py b/tools/telemetry/telemetry/value/__init__.py
index c4330ecdd92c5da1a3b24217ed9d5e43b2840694..ad30bf4cfe25ae741485b2e28f045fc131998480 100644
--- a/tools/telemetry/telemetry/value/__init__.py
+++ b/tools/telemetry/telemetry/value/__init__.py
@@ -132,8 +132,7 @@ class Value(object):
 
   def GetChartAndTraceNameForPerPageResult(self):
     chart_name, _ = _ConvertValueNameToChartAndTraceName(self.name)
-    trace_name = self.page.display_name
-    return chart_name, trace_name
+    return chart_name, self.page.display_name
 
   @property
   def name_suffix(self):
@@ -274,6 +273,10 @@ class Value(object):
 
     return d
 
+  def GetAssociatedFileHandle(self):
+    """Returns the file handle associated with this value (may be None)."""
+    return None
+
 def ValueNameFromTraceAndChartName(trace_name, chart_name=None):
   """Mangles a trace name plus optional chart name into a standard string.