Make ONCCertificateImporter async.

Make ONCCertificateImporter async.

This prepares for the new CertDatabase keyed service, which will have stricter threading restrictions. https://codereview.chromium.org/419013003/
Before, ONCCertificateImporter accessed the NSSCertDatabase from the UI thread and blocked on certificate store operations.
Now, the import itself is asychronous and calls back on completion.

While there, this also removes the GMock of the importer.

This is a reland of f08303014b165f6013fe33198cd798ebd9a4e925
  refs/heads/master@{#295534}
with the fixed destruction order in ONCCertificateImporterImplTest.

The fix was reviewed in
  https://codereview.chromium.org/589443002/

TBR=eroman@chromium.org
BUG=413219
Committed: https://crrev.com/3b4ba221657f6b27e2156818bc445c885d87fc0e
Cr-Commit-Position: refs/heads/master@{#295687}

[pneubeck (no reviews), 2014-09-19 10:06:36]

The CQ bit was checked by pneubeck@chromium.org

[pneubeck (no reviews), 2014-09-19 10:07:19]

pneubeck@chromium.org changed reviewers:
+ eroman@chromium.org, joaodasilva@chromium.org

[pneubeck (no reviews), 2014-09-19 10:07:20]

Reland with the fix reviewed already by Joao.

[commit-bot: I haz the power, 2014-09-19 10:07:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/582413002/1

[commit-bot: I haz the power, 2014-09-19 11:05:56]

Committed patchset #1 (id:1) as 203906e1d566f1eb96439fa83dd2ec7393a62715

[commit-bot: I haz the power, 2014-09-19 11:06:46]

Patchset 1 (id:??) landed as
https://crrev.com/3b4ba221657f6b27e2156818bc445c885d87fc0e
Cr-Commit-Position: refs/heads/master@{#295687}

[eugenis, 2014-09-19 14:30:52]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/584923002/ by eugenis@chromium.org.

The reason for reverting is: Use-after-free.
https://code.google.com/p/chromium/issues/detail?id=415916.