Extension API enterprise.platformKeys.

      First version that implements all proposed functions backed by the TPM; enabled on dev-channel only.

This is still missing several features:
- Reusing algorithm normalization of WebCrypto.
- Allow calling Sign() at most once per key.
- Storing which extension imported a certificate.
- Device-wide token.
- Passing publicExponent to generateKey.
- Throwing DOMExceptions instead of Errors.
- ...

BUG=364435

Review URL: https://codereview.chromium.org/214863002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272613 0039d316-1c4b-4281-b951-d872f2087c98