[webcrypto] Fix AES-KW unwrapping for symmetric keys (NSS).

The unwrapped key did not have the proper flags so it could not be used in other operations (like signing).

BUG=370712,245025

Review URL: https://codereview.chromium.org/269313004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269262 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed