Revert of [Telemetry] Change how trace_data detects if data being added is valid.

telemetry/telemetry/timeline/trace_data.py

Index: telemetry/telemetry/timeline/trace_data.py
diff --git a/telemetry/telemetry/timeline/trace_data.py b/telemetry/telemetry/timeline/trace_data.py
index e772af46f7d178342cba069fd460225d2aea40b8..bda70fc76a9b512250cb0896b5d14bfab3f969df 100644
--- a/telemetry/telemetry/timeline/trace_data.py
+++ b/telemetry/telemetry/timeline/trace_data.py
@@ -185,7 +185,7 @@
     self._raw_data = None
     return data
 
-  def AddEventsTo(self, part, events):
+  def AddEventsTo(self, part, events, as_string=False):
     """Note: this won't work when called from multiple browsers.
 
     Each browser's trace_event_impl zeros its timestamps when it writes them
@@ -195,15 +195,12 @@
     if self._raw_data == None:
       raise Exception('Already called AsData() on this builder.')
 
-    if isinstance(events, basestring):
-      if part.raw_field_name in self._raw_data:
-        self._raw_data[part.raw_field_name] += events
-      else:
-        self._raw_data[part.raw_field_name] = events
-    elif isinstance(events, list):
+    if as_string:
+      assert isinstance(events, basestring)
+      self._raw_data[part.raw_field_name] = events
+    else:
+      assert isinstance(events, list)
       self._raw_data.setdefault(part.raw_field_name, []).extend(events)
-    else:
-      raise TypeError('Trace event of unkown type being added. %s' % events)
 
   def HasEventsFor(self, part):
     return _HasEventsFor(part, self._raw_data)