Resubmit: Add a Restore() method to ValueStore and make StorageAPI use it

Resubmit of: https://codereview.chromium.org/165223003/

TBR=mpcomplete@chromium.org
TBR=joaodasilva@chromium.org

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Add a Restore() method to ValueStore and make StorageAPI use it
Take the first steps in preventing the Storage from getting and staying
corrupted. This lets us Restore() a ValueStore when it's corrputed, and tries to
do so in the StorageAPI calls.
This also refactors so that there's a Local and Sync SettingsBackend - this has
the added advantage of meaning we don't decorate the Local stores with Sync
behavior.
BUG=261623

Review URL: https://codereview.chromium.org/176963006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253353 0039d316-1c4b-4281-b951-d872f2087c98
37 files changed