Revert of Don't use RSAPrivateKey in NSS integration code.

Revert of Don't use RSAPrivateKey in NSS integration code. (patchset #6 id:100001 of https://codereview.chromium.org/1106103003/)

Reason for revert:
Causes SEGV during login on Chrome OS

BUG=483606
Original issue's description:
> Don't use RSAPrivateKey in NSS integration code.
>
> Currently some NSS platform integration logic transits private keys through
> RSAPrivateKey on CrOS. This prevents incrementally switching RSAPrivateKey to
> BoringSSL while keeping platform integrations on NSS.
>
> The intent of this change is to clarify RSAPrivateKey as a BoringSSL vs NSS
> internal crypto library (use_openssl=0 vs use_openssl=1) abstraction. It's
> primarily to be used with SignatureCreator. Code which uses NSS based on
> use_nss_certs rather than use_openssl because the underlying platform is NSS
> should call NSS routines directly, or introduce different abstractions.
>
> Remove the problematic RSAPrivateKey methods and instead add
> crypto/nss_key_util.h which contains some helper functions for manipulating NSS
> keys. This is sufficient to allow consumers of the removed methods to use NSS
> directly with about as much code. (This should not set back migrating that
> logic to NSS as that code was already very NSS-specific; those APIs assumed
> PK11SlotInfo.)
>
> nss_key_util.h, like nss_util.h, is built whenever NSS is used either
> internally or for platform integrations. This is so rsa_private_key_nss.cc can
> continue to use the helper functions to implement the NSS-agnostic interface.
>
> With this, the chimera CrOS configuration should build. The RSAPrivateKey logic
> is functional with the exception of some logic in components/ownership. That
> will be resolved in a future CL.
>
> BUG=478777
>
> Committed: https://crrev.com/a46a990b2ccae2b66e87b5f76d2866044dc3182e
> Cr-Commit-Position: refs/heads/master@{#327909}

TBR=rsleevi@chromium.org,pneubeck@chromium.org,dpolukhin@chromium.org,caitkp@chromium.org,davidben@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=483606
Committed: https://crrev.com/9ce3458d9a03b38ca717291d08d564e12fd8254a
Cr-Commit-Position: refs/heads/master@{#327978}

[spang, 2015-05-01 21:00:23]

Created Revert of Don't use RSAPrivateKey in NSS integration code.

[commit-bot: I haz the power, 2015-05-01 21:00:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1118263003/1

[spang, 2015-05-01 21:01:41]

The CQ bit was checked by spang@chromium.org

[spang, 2015-05-01 21:01:41]

The CQ bit was unchecked by spang@chromium.org

[commit-bot: I haz the power, 2015-05-01 21:02:11]

Committed patchset #1 (id:1)

[spang, 2015-05-01 21:02:16]

The CQ bit was checked by spang@chromium.org

[commit-bot: I haz the power, 2015-05-01 21:03:11]

Patchset 1 (id:??) landed as
https://crrev.com/9ce3458d9a03b38ca717291d08d564e12fd8254a
Cr-Commit-Position: refs/heads/master@{#327978}

[commit-bot: I haz the power, 2015-05-01 21:03:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1118263003/1