Revert of Use ExtensionRegistryObserver

Revert of Use ExtensionRegistryObserver instead of deprecated extension notification from autocomplete (https://codereview.chromium.org/271393003/)

Reason for revert:
Crash is reported.
See https://code.google.com/p/chromium/issues/detail?id=373624

Original issue's description:
> Use extensions::ExtensionRegistryObserver instead of deprecated extension notification from autocomplete
> 
> R=pkasting@chromium.org
> CC=kalman@chromium.org
> BUG=354046, 354458
> TEST=unit_tests
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=270268

TBR=pkasting@chromium.org,kalman@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=354046, 354458
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=270764

[limasdf, 2014-05-15 18:31:30]

Created Revert of Use ExtensionRegistryObserver instead of deprecated extension
notification from autocomplete

[commit-bot: I haz the power, 2014-05-15 18:33:22]

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

[Peter Kasting, 2014-05-15 18:33:39]

RSLGTM

[Peter Kasting, 2014-05-15 18:33:54]

The CQ bit was checked by pkasting@chromium.org

[limasdf, 2014-05-15 18:34:44]

On 2014/05/15 18:33:54, Peter Kasting wrote:
> The CQ bit was checked by mailto:pkasting@chromium.org

sorry to bother you, I'll investigate my previous patch.

[commit-bot: I haz the power, 2014-05-15 18:36:18]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/limasdf@gmail.com/290043003/1

[commit-bot: I haz the power, 2014-05-15 19:03:45]

Change committed as 270764