Added GPU performance metrics.

tools/telemetry/telemetry/web_perf/timeline_interaction_record.py

Index: tools/telemetry/telemetry/web_perf/timeline_interaction_record.py
diff --git a/tools/telemetry/telemetry/web_perf/timeline_interaction_record.py b/tools/telemetry/telemetry/web_perf/timeline_interaction_record.py
index 4c3462f16515dc2d2bccbd3f08254caf22809ff7..8c1c8170317329a2e3f10587cd732138064d8e77 100644
--- a/tools/telemetry/telemetry/web_perf/timeline_interaction_record.py
+++ b/tools/telemetry/telemetry/web_perf/timeline_interaction_record.py
@@ -176,6 +176,10 @@ class TimelineInteractionRecord(object):
                            'interaction record: %s' % metric_type)
     return metric_type in self._flags
 
+  def GetUserDefinedFlags(self):
+    return [metric_type for metric_type in METRICS
+            if metric_type in self._flags]
+
   def GetOverlappedThreadTimeForSlice(self, timeline_slice):
     """Get the thread duration of timeline_slice that overlaps with this record.