Remove SettingsBackend

Combine LocalStorageBackend and LocalValueStoreCache.
Remove SettingsBackend.

Sadly, SyncStorageBackend still exists.  It's significantly more tangled than
the local version, because we also keep apps/extensions separate when we sync
them.

BUG=351412

Review URL: https://codereview.chromium.org/193133002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256594 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256594