telemetry: Make trace profiler work with trace-based benchmarks.

tools/perf/measurements/smoothness.py

Index: tools/perf/measurements/smoothness.py
diff --git a/tools/perf/measurements/smoothness.py b/tools/perf/measurements/smoothness.py
index a26ba682913a7d981a11cc3b295fab93a717b7db..88b1f10baf62b8c53dc1621187ad7baf23789a85 100644
--- a/tools/perf/measurements/smoothness.py
+++ b/tools/perf/measurements/smoothness.py
@@ -43,9 +43,11 @@ class Smoothness(page_measurement.PageMeasurement):
     return hasattr(page, 'smoothness')
 
   def WillRunAction(self, page, tab, action):
-    # TODO(ernstm): remove 'webkit' category when
-    # https://codereview.chromium.org/23848006/ has landed.
-    tab.browser.StartTracing('webkit,webkit.console,benchmark', 60)
+    # We don't need to start tracing, if a trace profiler is already running.
+    # Note that all trace event categories are enabled in this case, which
+    # increases the risk of a trace buffer overflow.
+    if self.options.profiler != 'trace':
+      tab.browser.StartTracing('webkit.console,benchmark', 60)
     if tab.browser.platform.IsRawDisplayFrameRateSupported():
       tab.browser.platform.StartRawDisplayFrameRateMeasurement()
     self._metrics = smoothness.SmoothnessMetrics(tab)
@@ -59,6 +61,8 @@ class Smoothness(page_measurement.PageMeasurement):
       tab.browser.platform.StopRawDisplayFrameRateMeasurement()
     if not action.CanBeBound():
       self._metrics.Stop()
+    # Always stop tracing at this point. If a trace profiler is running, it will
+    # be stopped here.
     tab.browser.StopTracing()
 
   def FindTimelineMarker(self, timeline):
@@ -80,7 +84,12 @@ class Smoothness(page_measurement.PageMeasurement):
 
     smoothness.CalcFirstPaintTimeResults(results, tab)
 
-    timeline = tab.browser.GetTraceResultAndReset().AsTimelineModel()
+    # Fetch the results without resetting, if a trace profiler is running. The
+    # profiler needs to read the results later on.
+    if self.options.profiler == 'trace':
+      timeline = tab.browser.GetTraceResult().AsTimelineModel()
+    else:
+      timeline = tab.browser.GetTraceResultAndReset().AsTimelineModel()
     timeline_marker = self.FindTimelineMarker(timeline)
     benchmark_stats = GpuRenderingStats(timeline_marker,
                                         rendering_stats_deltas,