[webcrypto] Fix a bug where generated RSA private keys couldn't be exported or cloned.

BUG=245025
R=rsleevi@chromium.org, wtc@chromium.org

Review URL: https://codereview.chromium.org/213423007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260100 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed