[Extensions UI] Add tests for disabling/enabling/uninstalling extensions

[Extensions UI] Add tests for disabling/enabling/uninstalling extensions

Add some tests for disabling/enabling/uninstalling extensions from the
chrome://extensions page. Since we don't have any, these will help us determine
if we break anything during the transition to observing events from the
developerPrivate API.

BUG=461039
Committed: https://crrev.com/8363aa5def44e7942b582195e015e05570efae1e
Cr-Commit-Position: refs/heads/master@{#324076}

[Devlin, 2015-04-06 23:45:56]

Patchset #1 (id:1) has been deleted

[Devlin, 2015-04-06 23:51:14]

rdevlin.cronin@chromium.org changed reviewers:
+ dbeam@chromium.org

[Devlin, 2015-04-06 23:51:14]

Per request, tests have been extracted to this CL.

[Dan Beam, 2015-04-06 23:56:48]

first round of comments is here:
https://codereview.chromium.org/1049483003/#msg13

[Devlin, 2015-04-07 15:56:49]

Patchset #2 (id:40001) has been deleted

[Devlin, 2015-04-07 15:57:12]

On 2015/04/06 23:56:48, Dan Beam wrote:
> first round of comments is here:
> https://codereview.chromium.org/1049483003/#msg13

Done.

[Dan Beam, 2015-04-07 16:52:46]

lgtm

[Devlin, 2015-04-07 17:31:35]

The CQ bit was checked by rdevlin.cronin@chromium.org

[commit-bot: I haz the power, 2015-04-07 17:31:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1063863004/60001

[commit-bot: I haz the power, 2015-04-07 18:30:14]

Committed patchset #2 (id:60001)

[commit-bot: I haz the power, 2015-04-07 18:31:48]

Patchset 2 (id:??) landed as
https://crrev.com/8363aa5def44e7942b582195e015e05570efae1e
Cr-Commit-Position: refs/heads/master@{#324076}