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Creates a CryptographerProvider interface that can be used to expose the
syncable::Directory's Cryptographer without exposing the directory
itself.  Since the directory's cryptographer is protected by a lock,
this requires the creation of several helper classes to manage the
acquistion of the lock.

This CL includes the interface, an implementationthat exposes the
directory's cryptographer (DirectoryCryptographerProvider) and
a SimpleCryptographerProvider that wraps a non-owned Cryptographer for
testing purposes.

These interfaces will be used to implement and test encryption of
non-blocking sync types.

BUG=351005

Review URL: https://codereview.chromium.org/413833002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285777 0039d316-1c4b-4281-b951-d872f2087c98