Fixed leak in StorageSchemaManifestHandlerTest.

Fixed leak in StorageSchemaManifestHandlerTest.

This test uses the global ManifestHandlerRegistry, and its result depend on
the order of execution of other tests. If any test before created a
ManagedValueStoreCache then the ManifestHandlerRegistry has a
StorageSchemaManifestHandler, which will try to validate the bad extension
manifests, and will make this test fail.

This temporary fix always registers a StorageSchemaManifestHandler, and the
test doesn't assume that the extension is always created anymore. The long
term fix is to make ScopedTestingManifestHandlerRegistry public so that tests
like this can have a scoped ManifestHandlerRegistry.

TBR=kalman@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=202461

[Joao da Silva, 2013-05-27 17:50:06]

Benjamin, I found this little issue with the global ManifestHandlerRegistry.
WDYT of making ScopedTestingManifestHandlerRegistry public for other tests like
this to use?

[commit-bot: I haz the power, 2013-05-27 17:50:16]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/joaodasilva@chromium.org/16077003/1

[commit-bot: I haz the power, 2013-05-27 19:57:27]

Change committed as 202461

[not at google - send to devlin, 2013-05-28 14:30:21]

lgtm