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Currently DeleteAndStartOver() is tested in memory storage (MemEnv)
supported by LevelDB for performance, but it seems preferable to test
it on real environment, that is, on disk storage because the recovery
process can be affected by characteristics of lower filesystem.

BUG=468926
TEST=content_unittests --gtest_filter=ServiceWorkerContextTest.DeleteAndStartOver

Review URL: https://codereview.chromium.org/1022363002

Cr-Commit-Position: refs/heads/master@{#322346}