Support for group license

Content keys in ECM v3 can now additionally be encrypted by group
entitlement keys.

common/ec_key.cc

@@ -62,6 +62,8 @@ std::string GetMessageDigest(const std::string& message,
     case widevine::HashAlgorithm::kUnspecified:
     case widevine::HashAlgorithm::kSha256:
       return widevine::Sha256_Hash(message);
+    case widevine::HashAlgorithm::kSha384:
+      return widevine::Sha384_Hash(message);
     case widevine::HashAlgorithm::kSha1:
       LOG(ERROR) << "Unexpected hash algorithm: "
                  << static_cast<int>(hash_algorithm);