PrefValueStore incorrectly DCHECK()s when handling change notification for unregistered pref.

Simplified PrefValueStore::NotifyPrefChanged.

Reason for the refactoring is the need to properly handle disappearing policies.
This situation arises when a managed preference is removed that only has
been registered in one pref store (for example local_store) the notification
is sent to all pref stores though but the profile pref store doesn't have
a default value for that pref and therefore ControllingPrefStoreForPref returns
INVALID_STORE which shouldn't be treated as an exceptional situation (DCHECKed).

BUG=71028
TEST=unit_tests --gtest_filter=*Pref* should give pretty good coverage.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=72787

[pastarmovj, 2011-01-27 10:50:47]

Please review. Bots are all happy now :)

[Mattias Nissler (ping if slow), 2011-01-27 12:41:48]

LGTM, thanks for fixing.