Telemetry: Add a functional test for InspectorBackendList.GetTabById().

This CL also exposes the property tabs on the class TabTestCase.

BUG=442546

Review URL: https://codereview.chromium.org/940153002

Cr-Commit-Position: refs/heads/master@{#317662}

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

+# Copyright 2015 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 telemetry.core import exceptions
+from telemetry.unittest_util import tab_test_case
+
+
+class TabListBackendTest(tab_test_case.TabTestCase):
+  def testTabIdMatchesContextId(self):
+    # Ensure that there are two tabs.
+    while len(self.tabs) < 2:
+      self.tabs.New()
+
+    # Check that the tab.id matches context_id.
+    tabs = []
+    for context_id in self.tabs:
+      tab = self.tabs.GetTabById(context_id)
+      self.assertEquals(tab.id, context_id)
+      tabs.append(self.tabs.GetTabById(context_id))
+
+  def testTabIdStableAfterTabCrash(self):
+    # Ensure that there are two tabs.
+    while len(self.tabs) < 2:
+      self.tabs.New()
+
+    tabs = []
+    for context_id in self.tabs:
+      tabs.append(self.tabs.GetTabById(context_id))
+
+    # Crash the first tab.
+    self.assertRaises(exceptions.DevtoolsTargetCrashException,
+        lambda: tabs[0].Navigate('chrome://crash'))
+
+    # Fetching the second tab by id should still work. Fetching the first tab
+    # should raise an exception.
+    self.assertEquals(tabs[1], self.tabs.GetTabById(tabs[1].id))
+    self.assertRaises(KeyError, lambda: self.tabs.GetTabById(tabs[0].id))

tools/telemetry/telemetry/unittest_util/tab_test_case.py

@@ -40,3 +40,7 @@ class TabTestCase(browser_test_case.BrowserTestCase):
       self.tearDownClass()
       self.setUpClass()
     self._tab = self._browser.tabs[0]
+
+  @property
+  def tabs(self):
+    return self._browser.tabs