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

tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py

Index: tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py b/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
index 4d0c68a8a3edb617314407f60d0a8624407e4a14..b367e96504406907fac53d84c8366364c5ad949b 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
@@ -72,6 +72,9 @@ class TracingBackend(object):
     self._thread = None
     self._tracing_data = []
 
+  def _IsTracing(self):
+    return self._thread != None
+
   def BeginTracing(self, custom_categories=None, timeout=10):
     self._CheckNotificationSupported()
     req = {'method': 'Tracing.start'}
@@ -84,20 +87,26 @@ class TracingBackend(object):
     self._thread.start()
 
   def EndTracing(self):
+    if not self._IsTracing():
+      return
     req = {'method': 'Tracing.end'}
     self._conn.SendRequest(req)
     self._thread.join()
     self._thread = None
 
-  def GetTraceResultAndReset(self):
+  def GetTraceResult(self):
     assert not self._thread
     if self._tracing_data and type(self._tracing_data[0]) in [str, unicode]:
       result_impl = TraceResultImpl(self._tracing_data)
     else:
       result_impl = RawTraceResultImpl(self._tracing_data)
-    self._tracing_data = []
     return trace_result.TraceResult(result_impl)
 
+  def GetTraceResultAndReset(self):
+    result = self.GetTraceResult()
+    self._tracing_data = []
+    return result
+
   def _TracingReader(self):
     while self._conn.socket:
       try: