Implement chrome.platformKeys.getKeyPair().

This also makes the chrome.platformKeys.subtleCrypto().sign() working.
Still missing: per extension key permissions and UI changes.

Reland of
https://codereview.chromium.org/884073002/

BUG=450167
TBR=arv@chromium.org (for renderer_resources.grd change)

Review URL: https://codereview.chromium.org/927883002

Cr-Commit-Position: refs/heads/master@{#316410}
16 files changed