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 ba35cc1459152e77e151669b94df8cab5fb14963..95854c36057d6af8a293ecff6d3435d1fecf384f 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/tab_list_backend.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/tab_list_backend.py
@@ -4,6 +4,7 @@
 
 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
@@ -15,13 +16,29 @@ class TabListBackend(inspector_backend_list.InspectorBackendList):
   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.
@@ -36,6 +53,23 @@ class TabListBackend(inspector_backend_list.InspectorBackendList):
       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