Split up extensions storage API implementations for sync and local storage.

This will allow the core storage API implementation to move to //extensions where app_shell can use it.

* Split SyncOrLocalValueStoreCache into SyncValueStoreCache and LocalValueStoreCache
* Move SettingsFrontend::GetBackendForSync() to settings_sync_util::GetSyncableService()

BUG=348058
TEST=unit_tests *Settings* and browser_tests ExtensionSettings*
TBR=zea@chromium.org for trivial change to chrome/browser/sync/profile_sync_components_factory_impl.cc

Review URL: https://codereview.chromium.org/184043031

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255515 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255515