[telemetry] Add TraceResult as return value from tracing, and placeholder trace_event_importer.

tools/telemetry/telemetry/browser_backend.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import urllib2
 import httplib
 import socket
 import json
 import re
 import sys
@@ skipping 120 matching lines @@
     return True
 
   def StartTracing(self):
     if self._tracing_backend is None:
       self._tracing_backend = tracing_backend.TracingBackend(self._port)
     self._tracing_backend.BeginTracing()
 
   def StopTracing(self):
     self._tracing_backend.EndTracing()
 
-  def GetTrace(self):
+  def GetTraceResultAndReset(self):
     return self._tracing_backend.GetTraceAndReset()
 
   def GetRemotePort(self, _):
     return util.GetAvailableLocalPort()
 
   def Close(self):
     if self._tracing_backend:
       self._tracing_backend.Close()
       self._tracing_backend = None
 
   def CreateForwarder(self, *port_pairs):
     raise NotImplementedError()
 
   def IsBrowserRunning(self):
     raise NotImplementedError()
 
   def GetStandardOutput(self):
     raise NotImplementedError()