cleanup: Use ExtensionRegistryObserver in Uber

cleanup: Use ExtensionRegistryObserver in Uber

Use ExtensionRegistryObserver in Uber
since ExtensionRegistry replaces chrome::extension notifications.

BUG=411568
R=estade@chromium.org
TEST=browser_test --gtest_filter=UberUIBrowserTest.HistoryOverride

Committed: https://crrev.com/2166dd4a75f25f000dd10deecfecf0da3980b273
Cr-Commit-Position: refs/heads/master@{#322281}

[limasdf, 2015-03-25 23:20:04]

Resubmit: original is https://codereview.chromium.org/321563007/

Hi. can I get LGTM again? Previous CL is created 9 month ago. so it doesn't have
propfer field to submit. (crbug.com/470389)

[Evan Stade, 2015-03-25 23:22:44]

lgtm

[limasdf, 2015-03-25 23:23:07]

The CQ bit was checked by limasdf@gmail.com

[commit-bot: I haz the power, 2015-03-25 23:23:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1012773010/1

[commit-bot: I haz the power, 2015-03-26 00:55:47]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-26 00:56:55]

Patchset 1 (id:??) landed as
https://crrev.com/2166dd4a75f25f000dd10deecfecf0da3980b273
Cr-Commit-Position: refs/heads/master@{#322281}