[Telemetry] Factor out common logic between inspector backend lists.

tools/telemetry/telemetry/core/extension_unittest.py

Index: tools/telemetry/telemetry/core/extension_unittest.py
diff --git a/tools/telemetry/telemetry/core/extension_unittest.py b/tools/telemetry/telemetry/core/extension_unittest.py
index bc5fd78ded9ab7e69a92fe78338af99f014f7bf7..f17d029a0549294ad8de4326ce8f54263397759e 100644
--- a/tools/telemetry/telemetry/core/extension_unittest.py
+++ b/tools/telemetry/telemetry/core/extension_unittest.py
@@ -11,7 +11,6 @@ import unittest
 from telemetry.core import browser_finder
 from telemetry.core import extension_to_load
 from telemetry.core import util
-from telemetry.core.backends.chrome import extension_dict_backend
 from telemetry.unittest import options_for_unittests
 
 
@@ -69,8 +68,7 @@ class NonExistentExtensionTest(unittest.TestCase):
     browser_to_create = browser_finder.FindBrowser(options)
     with browser_to_create.Create() as b:
       if b.supports_extensions:
-        self.assertRaises(extension_dict_backend.ExtensionNotFoundException,
-                          lambda: b.extensions[load_extension])
+        self.assertRaises(KeyError, lambda: b.extensions[load_extension])
 
 class MultipleExtensionTest(unittest.TestCase):
   def setUp(self):