Add support for new calls in cryptohome.

Related changes on chromeos side:

https://chromium-review.googlesource.com/#/c/186561/
https://chromium-review.googlesource.com/#/c/188490/

New methods added: MountEx, CheckKeyEx, AddKeyEx, UpdateKeyEx (extended versions of Mount, CheckKey, AddKey calls).

This is done to achieve several goals. One is to unify most of cryptohome calls by using protobuffers.
Using protobuffers also makes most of the code resilient to implementation changes.
These methods also introduce key annotations (labels), that are used to specify keys for operations on valuts with multiple keys, as well as introduce per-key privileges.

First part of CL: https://chromiumcodereview.appspot.com/186663002/
NOTRY=true
as it only adds code.
BUG=244472

Review URL: https://codereview.chromium.org/198003002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256823 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256823