Make ServiceWorkerDatabase::DestroyDatabase() work on in-memory db

Make ServiceWorkerDatabase::DestroyDatabase() work on in-memory db

When |path_| is empty, MemEnv created in LazyOpen() should be passed to
leveldb::DestroyDB. Otherwise, on Windows lebeldb tries to delete all
files in current directory that look like leveldb files.

This should fix ServiceWorkerContextTest.DeleteAndStartOver test on
Windows, so re-enable it.

BUG=420124
TEST=content_unittests --gtest_filter=ServiceWorkerContextTest.*

Committed: https://crrev.com/939d5288a99e0762b96ed38e456d26499470d891
Cr-Commit-Position: refs/heads/master@{#299874}

[Kunihiko Sakamoto, 2014-10-16 09:38:02]

Patchset #1 (id:1) has been deleted

[Kunihiko Sakamoto, 2014-10-16 09:48:32]

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

[nhiroki, 2014-10-16 10:02:26]

Good catch!! LGTM

[Kunihiko Sakamoto, 2014-10-16 10:10:48]

The CQ bit was checked by ksakamoto@chromium.org

[commit-bot: I haz the power, 2014-10-16 10:11:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/651133004/20001

[commit-bot: I haz the power, 2014-10-16 10:24:41]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2014-10-16 10:25:21]

Patchset 1 (id:??) landed as
https://crrev.com/939d5288a99e0762b96ed38e456d26499470d891
Cr-Commit-Position: refs/heads/master@{#299874}