Improve error reporting when indexing tab.

tools/telemetry/telemetry/core/tab_unittest.py

Index: tools/telemetry/telemetry/core/tab_unittest.py
diff --git a/tools/telemetry/telemetry/core/tab_unittest.py b/tools/telemetry/telemetry/core/tab_unittest.py
index 6f1389ade9be07baf89d742b5d73cf057c483cb9..f99191bcdcc099a4880c7df4017e949951014971 100644
--- a/tools/telemetry/telemetry/core/tab_unittest.py
+++ b/tools/telemetry/telemetry/core/tab_unittest.py
@@ -65,6 +65,23 @@ class TabTest(tab_test_case.TabTestCase):
     self.assertRaises(exceptions.TabCrashException,
                       lambda: self._tab.Navigate('chrome://crash',
                                                  timeout=5))
+    super(TabTest, self).tearDownClass()
+    super(TabTest, self).setUpClass()
+
+  def testIndexTab(self):
+    self._browser.tabs.New()
+    self.assertRaises(exceptions.TabCrashException,
+                      lambda: self._tab.Navigate('chrome://crash',
+                                               timeout=5))
+    self.assertRaises(exceptions.TabIndexError,
+                      lambda: self._browser.tabs[2])
+    self.assertRaises(exceptions.TabCrashException,
+                      lambda: self._browser.tabs[0])
+    self.assertRaises(exceptions.TabCrashException,
+                      lambda: self._browser.tabs[1])
+    self._browser.Close()
+    super(TabTest, self).tearDownClass()
+    super(TabTest, self).setUpClass()
 
   @benchmark.Enabled('has tabs')
   def testActivateTab(self):