Remove the weak dictionary that maps tab objects to it markers

tools/telemetry/telemetry/web_perf/timeline_based_measurement.py

 # Copyright 2014 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 logging
 import os
 
 from telemetry.core import util
 from telemetry.core.backends.chrome import tracing_backend
 from telemetry.core.timeline import model as model_module
@@ skipping 133 matching lines @@
           os.path.join(trace_dir, 'trace')) + '.json'
       try:
         with open(trace_file_path, 'w') as f:
           trace_result.Serialize(f)
         results.AddValue(string_value_module.StringValue(
             page, 'trace_path', 'string', trace_file_path))
       except IOError, e:
         logging.error('Cannot open %s. %s' % (trace_file_path, e))
 
     model = model_module.TimelineModel(trace_result)
-    renderer_thread = model.GetRendererThreadFromTab(tab)
+    renderer_thread = model.GetRendererThreadFromTabId(tab.id)
     meta_metrics = _TimelineBasedMetrics(
       model, renderer_thread, self.CreateMetricsForTimelineInteractionRecord)
     meta_metrics.AddResults(results)
 
   def CleanUpAfterPage(self, page, tab):
     if tab.browser.is_tracing_running:
       tab.browser.StopTracing()