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

tools/telemetry/telemetry/page_runner.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 codecs
 import logging
 import os
 import time
 import traceback
 import urlparse
 import random
@@ skipping 230 matching lines @@
   def _SetupTracingTab(self, state):
     if state.browser.supports_tracing:
       state.is_tracing = True
       state.browser.StartTracing()
 
   def _EndTracing(self, state, options, page):
     if state.is_tracing:
       assert state.browser
       state.is_tracing = False
       state.browser.StopTracing()
-      trace = state.browser.GetTrace()
+      trace_result = state.browser.GetTraceResultAndReset()
       logging.info('Processing trace...')
 
       trace_file_base = os.path.join(
           options.trace_dir, page.url_as_file_safe_name)
 
       if options.page_repeat != 1 or options.pageset_repeat != 1:
         trace_file_index = 0
 
         while True:
           trace_file = '%s_%03d.json' % (trace_file_base, trace_file_index)
           if not os.path.exists(trace_file):
             break
           trace_file_index = trace_file_index + 1
       else:
         trace_file = '%s.json' % trace_file_base
       with codecs.open(trace_file, 'w',
                        encoding='utf-8') as trace_file:
-        trace_file.write(trace)
+        trace_result.WriteToFile(trace_file)
       logging.info('Trace saved.')
 
   def _PreparePage(self, page, tab, page_state, test, results):
     parsed_url = urlparse.urlparse(page.url)
     if parsed_url[0] == 'file':
       dirname, filename = page.url_base_dir_and_file
       tab.browser.SetHTTPServerDirectory(dirname)
       target_side_url = tab.browser.http_server.UrlOf(filename)
     else:
       target_side_url = page.url
@@ skipping 25 matching lines @@
       tab.browser.credentials.LoginNoLongerNeeded(tab, page.credentials)
     try:
       tab.EvaluateJavaScript("""window.chrome && chrome.benchmarking &&
                               chrome.benchmarking.closeConnections()""")
     except Exception:
       pass
 
   @staticmethod
   def AddCommandLineOptions(parser):
     page_filter_module.PageFilter.AddCommandLineOptions(parser)