[GCrypt] Implement AES_CBC support

[GCrypt] Implement AES_CBC support
https://bugs.webkit.org/show_bug.cgi?id=170550

Reviewed by Michael Catanzaro.

Implement the CryptoAlgorithmAES_CBC::platform{Encrypt,Decrypt}
functionality for configurations that use libgcrypt. This is done
by leveraging the gcry_cipher_* APIs for the AES algorithm that's
deduced appropriately from the key size and the CBC cipher mode.
Additionally, the PKCS#7 padding is implemented for each operation,
as demanded by the Web Crypto specification.

No new tests -- current ones cover this sufficiently, but are not yet
enabled due to missing platform-specific SUBTLE_CRYPTO implementations.

* crypto/gcrypt/CryptoAlgorithmAES_CBCGCrypt.cpp:
(WebCore::gcryptEncrypt):
(WebCore::gcryptDecrypt):
(WebCore::CryptoAlgorithmAES_CBC::platformEncrypt):
(WebCore::CryptoAlgorithmAES_CBC::platformDecrypt):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@215087 268f45cc-cd09-0410-ab3c-d52691b4dbfc