[telemetry] Add a webdriver backend with support for IE.

tools/telemetry/telemetry/core/backends/webdriver/webdriver_tab_list_backend.py

+# Copyright (c) 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.
+
+from telemetry.core.backends.webdriver import webdriver_tab_backend
+
+class WebDriverTabListBackend(object):
+  def __init__(self, browser_backend):
+    self._browser_backend = browser_backend
+    # Stores the window handles.
+    self._tab_list = []
+
+  def Init(self):
+    self._UpdateTabList()
+
+  def New(self, timeout=None):
+    # Webdriver API doesn't support tab controlling.
+    raise NotImplementedError()
+
+  def __iter__(self):
+    self._UpdateTabList()
+    return self._tab_list.__iter__()
+
+  def __len__(self):
+    self._UpdateTabList()
+    return len(self._tab_list)
+
+  def __getitem__(self, index):
+    self._UpdateTabList()
+    if len(self._tab_list) <= index:
+      raise IndexError('list index out of range')
+    return self._tab_list[index]
+
+  def _UpdateTabList(self):
+    window_handles = self._browser_backend.driver.window_handles
+    old_tab_list = self._tab_list
+    self._tab_list = []
+    for window_handle in window_handles:
+      tab = None
+      for old_tab in old_tab_list:
+        if old_tab.window_handle == window_handle:
+          tab = old_tab
+          break
+      else:
+        tab = webdriver_tab_backend.WebDriverTabBackend(
+            self._browser_backend, window_handle)
+      self._tab_list.append(tab)