[webcrypto] Refactor to share more code between OpenSSL and NSS implementations.

BUG=245025

Review URL: https://codereview.chromium.org/155623005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251961 0039d316-1c4b-4281-b951-d872f2087c98
16 files changed