Implement chrome.platformKeys.getKeyPair().

This also makes the chrome.platformKeys.subtleCrypto().sign() working.

Second change after
https://codereview.chromium.org/875373002/
to implement chrome.platformKeys.

Still missing: per extension key permissions and UI changes.

BUG=450167
R=asvitkine@chromium.org, kaliamoorthi@chromium.org, kalman@chromium.org, rsleevi@chromium.org
TBR=arv@chromium.org (for .grd change)

Review URL: https://codereview.chromium.org/884073002

Cr-Commit-Position: refs/heads/master@{#316386}
23 files changed