[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..7c1e26c43d4eea27b7215e9e4b87a2287d731ccb 100644
--- a/tools/telemetry/telemetry/value/__init__.py
+++ b/tools/telemetry/telemetry/value/__init__.py
@@ -274,6 +274,10 @@ 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.