[sync] Auto-create credential cache for users who are already signed in and go on to upgrade Chrome

CredentialCacheService on Windows 8 writes to the local credential cache
when a user actively signs in / reconfigures sync. Existing cached
credentials are not updated when Chrome is merely restarted after the
user is signed in.

This causes a problem when a user is already signed in, and then
upgrades (and restarts) chrome from a version that didn't originally
support credential caching. In such cases, we never end up caching
credentials, and therefore, the user will have to sign in separately to
Metro and Desktop.

This patch contains the following changes:

1) Adds logic to auto-heal already-signed-in users who upgrade from
older versions, by writing existing credentials to the local cache if
during restart, we notice that there is no local cache file, and the
user is already signed in to sync.

2) Simplifies the logic around checking if an alternate credential cache
file exists, and only then initializing |alternate_store_|. It turns out
that JsonPrefStore returns a useful PrefReadError field, and there is no
need for CCS to do funky stuff on the FILE thread.

3) Simplifies OnInitialzationCompleted, which was being used to observe
two separate JsonPrefStores. Instead of having CCS be a
PrefStore::Observer, we now use two helper classes -- LocalStoreObserver
and AlternateStoreObserver to cleanly divide what is done when each pref
store is initialized.

4) Updates prefs::kGoogleServicesUsername by listening to the notifications
NOTIFICATION_GOOGLE_SIGNED_OUT and NOTIFICATION_GOOGLE_SIGNIN_SUCCESSFUL
instead of directly listening to the pref change.

5) Fixes a stray instance where we were accessing the gaia username
pref via SyncPrefs instead of via the SigninManager.

BUG=141555
TEST=Sign in to chrome, exit the browser, and delete "Sync Credentials" from the default profile directory. Restart Chrome and make sure that the credential cache file is freshly written using existing sync credentials.

Review URL: https://chromiumcodereview.appspot.com/10830239

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@151364 0039d316-1c4b-4281-b951-d872f2087c98

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