Convert smoothness to the new timeline based metric API.

tools/perf/metrics/timeline.py

Index: tools/perf/metrics/timeline.py
diff --git a/tools/perf/metrics/timeline.py b/tools/perf/metrics/timeline.py
index 9313d99fe6aba852c87659d12267f433f217ea21..89b1b7f336c799455c7a14ef5f6b374d4db5f1a1 100644
--- a/tools/perf/metrics/timeline.py
+++ b/tools/perf/metrics/timeline.py
@@ -51,7 +51,12 @@ class TimelineMetric(Metric):
     if self._mode == TRACING_MODE:
       if not tab.browser.supports_tracing:
         raise Exception('Not supported')
-      tab.browser.StartTracing(self.trace_categories)
+      if self.trace_categories:
+        categories = [self.trace_categories] + \
+            page.GetSyntheticDelayCategories()
+      else:
+        categories = page.GetSyntheticDelayCategories()
+      tab.browser.StartTracing(','.join(categories))
     else:
       assert self._mode == TIMELINE_MODE
       tab.StartTimelineRecording()