Migrate browsertest to use ServiceWorkerRegistration.unregister()

Migrate browsertest to use ServiceWorkerRegistration.unregister()

ServiceWorkerContainer.unregister() is going away. This patch
updates ServiceWorkerBrowserTest.ImportsBustMemcache to use
ServiceWorkerRegistration.unregister() function.

BUG=404064
Committed: https://crrev.com/4860e1362eb5541c160ef85ad1c75ce338752f6f
Cr-Commit-Position: refs/heads/master@{#295664}

[Kunihiko Sakamoto, 2014-09-18 11:03:02]

ksakamoto@chromium.org changed reviewers:
+ michaeln@chromium.org

[Kunihiko Sakamoto, 2014-09-19 05:00:10]

ksakamoto@chromium.org changed reviewers:
+ dominicc@chromium.org

[Kunihiko Sakamoto, 2014-09-19 05:00:10]

adding a reviewer in JST

[dominicc (has gone to gerrit), 2014-09-19 05:03:42]

The CQ bit was checked by dominicc@chromium.org

[dominicc (has gone to gerrit), 2014-09-19 05:03:42]

lgtm

[commit-bot: I haz the power, 2014-09-19 05:04:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/579183002/1

[commit-bot: I haz the power, 2014-09-19 05:29:50]

Committed patchset #1 (id:1) as 6f506d6cb943cb2b67455eb2e8a31ec510ad065e

[commit-bot: I haz the power, 2014-09-19 05:30:27]

Patchset 1 (id:??) landed as
https://crrev.com/4860e1362eb5541c160ef85ad1c75ce338752f6f
Cr-Commit-Position: refs/heads/master@{#295664}