Fix ServiceWorkerResourceStorageDiskTest.CleanupOnRestart flakiness on Android

Fix ServiceWorkerResourceStorageDiskTest.CleanupOnRestart flakiness on Android

This test involves interactions with SimpleCache's worker thread,
so we cannot guarantee task completion by simply calling
RunLoop().RunUntilIdle().

TestCompletionCallback waits until the callback is called. For purging,
there is no completion callback so poll is_purge_pending_ flag and call
RunUntilIdle() repeatedly.

BUG=387045
TEST=content_unittests --gtest-filter=ServiceWorkerResourceStorageDiskTest.*

Committed: https://crrev.com/558a1c9d789332e0151c989963f6517b29455f4b
Cr-Commit-Position: refs/heads/master@{#300632}

[Kunihiko Sakamoto, 2014-10-21 07:42:18]

Patchset #1 (id:1) has been deleted

[Kunihiko Sakamoto, 2014-10-22 01:44:14]

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

[nhiroki, 2014-10-22 02:16:06]

drive-by lgtm

[Kunihiko Sakamoto, 2014-10-22 02:38:13]

The CQ bit was checked by ksakamoto@chromium.org

[commit-bot: I haz the power, 2014-10-22 02:38:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/657023012/20001

[commit-bot: I haz the power, 2014-10-22 04:18:47]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2014-10-22 04:19:36]

Patchset 1 (id:??) landed as
https://crrev.com/558a1c9d789332e0151c989963f6517b29455f4b
Cr-Commit-Position: refs/heads/master@{#300632}