Revert of Improve error reporting when indexing tab.

Revert of Improve error reporting when indexing tab. (patchset #7 of https://codereview.chromium.org/449753002/)

Reason for revert:
This patch broke chromeos bot in https://code.google.com/p/chromium/issues/detail?id=406010

A reland of this must include fixes in:
https://codereview.chromium.org/484353003/
https://codereview.chromium.org/499453002/

TBR=tonyg@chromium.org,kbr@chromium.org,jennyz@chromium.org,achuith@chromium.org
NOTRY=true
NOTREECHECKS=true
BUG=405955, 406010
Original issue's description:
> Improve error reporting when indexing tab.
> 
> If we created 3 new tabs, and 1 of them crashed, then there 
> is no guaranteed that  browser.tabs[1] return the correct tab 
> object. The tabs list can no longer be trusted in this case. Hence if 
> the number of inspectable contexts doesn't match the number 
> of expected tabs (based on the number of 
> tab_list_backend.New & tab_list_backend.Close calls), 
> tab_list_backend's __getitem__ throws a TabCrashException.
> 
> BUG=398467
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=290974

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=291431

[nednguyen, 2014-08-22 16:28:54]

Created Revert of Improve error reporting when indexing tab.

[commit-bot: I haz the power, 2014-08-22 16:29:13]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/nednguyen@google.com/494753003/1

[commit-bot: I haz the power, 2014-08-22 16:29:14]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-08-22 16:29:14]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer or
a provisional committer, _not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[nednguyen, 2014-08-22 16:31:06]

The CQ bit was checked by nednguyen@google.com

[commit-bot: I haz the power, 2014-08-22 16:35:00]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/nednguyen@google.com/494753003/1

[commit-bot: I haz the power, 2014-08-22 16:42:36]

Committed patchset #1 (1) as 291431

[achuithb, 2014-08-22 17:24:27]

lgtm