[webcrypto] Add key generation for AES-GCM and AES-KW for NSS.

BUG=245025
TEST=content_unittests --gtest_filter="WebCryptoImpl*"

Review URL: https://codereview.chromium.org/119413002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242255 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242255