Remove the weak dictionary that maps tab objects to it markers

tools/perf/measurements/loading_trace.py

 # Copyright 2013 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.
 from measurements import timeline_controller
 from metrics import loading
 from metrics import timeline
 from telemetry.page import page_measurement
 from telemetry.web_perf import timeline_interaction_record as tir_module
 
 class LoadingTrace(page_measurement.PageMeasurement):
@@ skipping 13 matching lines @@
     # but we need to wait for the load event.
     tab.WaitForJavaScriptExpression('performance.timing.loadEventStart', 300)
 
     # TODO(nduca): when crbug.com/168431 is fixed, modify the page sets to
     # recognize loading as a toplevel action.
     self._timeline_controller.Stop(tab)
 
     loading.LoadingMetric().AddResults(tab, results)
     timeline_metric = timeline.LoadTimesTimelineMetric()
     renderer_thread = \
-        self._timeline_controller.model.GetRendererThreadFromTab(tab)
+        self._timeline_controller.model.GetRendererThreadFromTabId(tab.id)
     record = tir_module.TimelineInteractionRecord(
       "loading_trace_interaction", 0, float('inf'))
     timeline_metric.AddResults(
       self._timeline_controller.model,
       renderer_thread,
       [record],
       results)
 
   def CleanUpAfterPage(self, _, tab):
     self._timeline_controller.CleanUp(tab)