Revert of Improve error reporting when indexing tab.

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

Index: tools/telemetry/telemetry/core/backends/chrome/tab_list_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/chrome/tab_list_backend.py b/tools/telemetry/telemetry/core/backends/chrome/tab_list_backend.py
index 95854c36057d6af8a293ecff6d3435d1fecf384f..ba35cc1459152e77e151669b94df8cab5fb14963 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/tab_list_backend.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/tab_list_backend.py
@@ -4,7 +4,6 @@
 
 import urllib2
 
-from telemetry.core import exceptions
 from telemetry.core import tab
 from telemetry.core import util
 from telemetry.core.backends.chrome import inspector_backend_list
@@ -16,29 +15,13 @@
   def __init__(self, browser_backend):
     super(TabListBackend, self).__init__(browser_backend,
                                          backend_wrapper=tab.Tab)
-    # This variable keeps track of the number of expected tabs based on the
-    # number of New() and Close() commands are called. This is different from
-    # the number of live tabs (__len__) which excludes crashed tabs.
-    # We initialize this to None because browser maybe started with more than
-    # 0 tabs, hence we set it to the number of live tabs after browser starts.
-    self._num_expected_tabs = None
-
-  def DidStartBrowser(self):
-    self._num_expected_tabs = len(self)
 
   def New(self, timeout):
     assert self._browser_backend.supports_tab_control
     self._browser_backend.Request('new', timeout=timeout)
-    self._num_expected_tabs += 1
     return self[-1]
 
   def CloseTab(self, debugger_url, timeout=None):
-    ''' Close tab.
-    This methods raises urllib2.HTTPError if tab id is not found. It may also
-    raise timeout error or network error. In this case, there is no guaranteed
-    on tab indexing logic, i.e: the tab may be closed, or maybe not, and the
-    number of expected tabs may no longer be correct.
-    '''
     assert self._browser_backend.supports_tab_control
     tab_id = inspector_backend_list.DebuggerUrlToId(debugger_url)
     # TODO(dtu): crbug.com/160946, allow closing the last tab on some platforms.
@@ -53,23 +36,6 @@
       raise Exception('Unable to close tab, tab id not found: %s' % tab_id)
     assert response == 'Target is closing'
     util.WaitFor(lambda: tab_id not in self, timeout=5)
-    self._num_expected_tabs -= 1
-    assert self._num_expected_tabs >= 0
-
-  def __getitem__(self, index):
-    if index >= self._num_expected_tabs:
-      raise exceptions.TabIndexError(
-          'Tab index error. Number of opening tabs is %i, while index is'
-          ' %i.' % (self._num_expected_tabs, index))
-    if self._num_expected_tabs > len(self):
-      raise exceptions.TabCrashException(
-          self._browser_backend.browser,
-          'Number of opening tabs is %i, whereas number of live tabs is %i, '
-          'Tried to get tab at index %i but this may not return the right tab.'
-          % (self._num_expected_tabs, len(self), index))
-    assert self._num_expected_tabs == len(self)
-
-    return super(TabListBackend, self).__getitem__(index)
 
   def ActivateTab(self, debugger_url, timeout=None):
     assert self._browser_backend.supports_tab_control