[Telemetry] Don't create a new Tab object at each navigation.

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

Index: tools/telemetry/telemetry/core/backends/chrome/extension_dict_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/chrome/extension_dict_backend.py b/tools/telemetry/telemetry/core/backends/chrome/extension_dict_backend.py
index bcde8a604c52da991440b6de5ede561c4bb469fa..7883be31e5daffd103ab0c56de9e8e6868d9b498 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/extension_dict_backend.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/extension_dict_backend.py
@@ -1,21 +1,23 @@
 # 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.
+
 import json
 import re
-import weakref
 
 from telemetry.core import extension_page
 from telemetry.core.backends.chrome import inspector_backend
 
+
 class ExtensionNotFoundException(Exception):
   pass
 
+
 class ExtensionDictBackend(object):
   def __init__(self, browser_backend):
     self._browser_backend = browser_backend
     # Maps extension ids to ExtensionPage objects.
-    self._extension_dict = weakref.WeakValueDictionary()
+    self._extension_dict = {}
 
   def __getitem__(self, extension_id):
     extension_object = self._extension_dict.get(extension_id)