[Telemetry]: Move assert on page measurement results name into Value object.

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 6148832b1414ef21c9d84feb02f71851636d1b6a..991b2492c0f7e68cf5483afc97320990409c258a 100644
--- a/tools/telemetry/telemetry/value/__init__.py
+++ b/tools/telemetry/telemetry/value/__init__.py
@@ -164,6 +164,8 @@ def ValueNameFromTraceAndChartName(trace_name, chart_name=None):
   if chart_name:
     return '%s.%s' % (chart_name, trace_name)
   else:
+    assert '.' not in trace_name, ('Trace names cannot contain "." with an '
+        'empty chart_name since this is used to delimit chart_name.trace_name.')
     return trace_name
 
 def _ConvertValueNameToBuildbotChartAndTraceName(value_name):