PhantomJS, arraybuffer and obsolete webkit version :(

test.js

var page = new WebPage();
page.open('http://www.google.fr/', function(status) {
    var base64 = page.evaluate(function() {
        function base64ArrayBuffer(arrayBuffer) {
            var base64        = ''
            var encodings     = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
            var bytes         = new Uint8Array(arrayBuffer)
            var byteLength    = bytes.byteLength
            var byteRemainder = byteLength % 3
            var mainLength    = byteLength - byteRemainder
            var a, b, c, d
            var chunk
            // Main loop deals with bytes in chunks of 3
            for (var i = 0; i < mainLength; i = i + 3) {
                // Combine the three bytes into a single integer
                chunk = (bytes[i] << 16) | (bytes[i + 1] << 8) | bytes[i + 2]
                // Use bitmasks to extract 6-bit segments from the triplet
                a = (chunk & 16515072) >> 18 // 16515072 = (2^6 - 1) << 18
                b = (chunk & 258048)     >> 12 // 258048     = (2^6 - 1) << 12
                c = (chunk & 4032)         >>    6 // 4032         = (2^6 - 1) << 6
                d = chunk & 63                             // 63             = 2^6 - 1
                // Convert the raw binary segments to the appropriate ASCII encoding
                base64 += encodings[a] + encodings[b] + encodings[c] + encodings[d]
            }
            // Deal with the remaining bytes and padding
            if (byteRemainder == 1) {
                chunk = bytes[mainLength]
                a = (chunk & 252) >> 2 // 252 = (2^6 - 1) << 2
                // Set the 4 least significant bits to zero
                b = (chunk & 3)     << 4 // 3     = 2^2 - 1
                base64 += encodings[a] + encodings[b] + '=='
            } else if (byteRemainder == 2) {
                chunk = (bytes[mainLength] << 8) | bytes[mainLength + 1]
                a = (chunk & 64512) >> 10 // 64512 = (2^6 - 1) << 10
                b = (chunk & 1008)    >>    4 // 1008    = (2^6 - 1) << 4
                // Set the 2 least significant bits to zero
                c = (chunk & 15)        <<    2 // 15        = 2^4 - 1
                base64 += encodings[a] + encodings[b] + encodings[c] + '='
            }
            return base64
        }
        var out, xhr = new XMLHttpRequest();
        xhr.open('GET', 'http://www.google.fr/images/srpr/logo3w.png', false);
        xhr.responseType = 'arraybuffer';
        xhr.onload = function(e) {
            try {
                out = base64ArrayBuffer(e.currentTarget.response);
            } catch (err) {
                out = err.message;
            }
        };
        xhr.send();
        return out;
    });
    console.log(base64);
    phantom.exit();
});