content-script.js

function signatureMethod (consumerSecret, tokenSecret, signatureBaseString) {
    var encoder = new TextEncoder('utf-8');
    var passphrase;

    consumerSecret = consumerSecret;
    tokenSecret = tokenSecret || '';
    passphrase = consumerSecret + '&' + tokenSecret;

    return crypto.subtle.importKey(
        "raw",
        encoder.encode(passphrase),
        {
            name: "HMAC",
            hash: {
                name: "SHA-1"
            }
        },
        false,
        ["sign", "verify"]
    ).then(function (key) {
        return crypto.subtle.sign(
            {
                name: "HMAC"
            },
            key,
            encoder.encode(signatureBaseString)
        );
    }).then(function (signature) {
        var result = [];
        (new Uint8Array(signature)).forEach(function (char) {
            result.push(String.fromCharCode(char));
        });
        return result.join('');
    }).catch(function (err) {
        console.error('Could not sign: ', err);
    });
}

signatureMethod("blah blah blah", "something", "long signature string to")
.then((response) => console.log("sign", response));