Fix for extension binding race condition when evaluating JS.

tools/telemetry/telemetry/unittest/tab_test_case.py

Index: tools/telemetry/telemetry/unittest/tab_test_case.py
diff --git a/tools/telemetry/telemetry/unittest/tab_test_case.py b/tools/telemetry/telemetry/unittest/tab_test_case.py
index 9ccd504f86a4f3079079e0fa38b632bb467fa13c..402109e472a1b0a992206d3a4f682132c97d943e 100644
--- a/tools/telemetry/telemetry/unittest/tab_test_case.py
+++ b/tools/telemetry/telemetry/unittest/tab_test_case.py
@@ -28,6 +28,8 @@ class TabTestCase(unittest.TestCase):
       self._browser = browser_to_create.Create()
       self._browser.Start()
       self._tab = self._browser.tabs[0]
+      self._tab.Navigate('about:blank')
+
     except:
       self.tearDown()
       raise