Base64 (btoa/atob) shim adapted for WebKit's Web Workers, which miss these functions. See https://bugs.webkit.org/show_bug.cgi?id=55663

base64shim.js

// Source: http://code.google.com/p/gflot/source/browse/trunk/flot/base64.js?r=153

/* Copyright (C) 1999 Masanao Izumo <[email protected]>
 * Version: 1.0
 * LastModified: Dec 25 1999
 * This library is free. You can redistribute it and/or modify it.
 */

/*
 * Interfaces:
 * b64 = base64encode(data);
 * data = base64decode(b64);
 */

(function() {

    var base64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    var base64DecodeChars = new Array(
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,
        52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1,
        -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
        15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,
        -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
        41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1);

    function base64encode(str) {
        var out, i, len;
        var c1, c2, c3;

        len = str.length;
        i = 0;
        out = "";
        while(i < len) {
            c1 = str.charCodeAt(i++) & 0xff;
            if(i == len)
            {
                out += base64EncodeChars.charAt(c1 >> 2);
                out += base64EncodeChars.charAt((c1 & 0x3) << 4);
                out += "==";
                break;
            }
            c2 = str.charCodeAt(i++);
            if(i == len)
            {
                out += base64EncodeChars.charAt(c1 >> 2);
                out += base64EncodeChars.charAt(((c1 & 0x3)<< 4) | ((c2 & 0xF0) >> 4));
                out += base64EncodeChars.charAt((c2 & 0xF) << 2);
                out += "=";
                break;
            }
            c3 = str.charCodeAt(i++);
            out += base64EncodeChars.charAt(c1 >> 2);
            out += base64EncodeChars.charAt(((c1 & 0x3)<< 4) | ((c2 & 0xF0) >> 4));
            out += base64EncodeChars.charAt(((c2 & 0xF) << 2) | ((c3 & 0xC0) >>6));
            out += base64EncodeChars.charAt(c3 & 0x3F);
        }
        return out;
    }

    function base64decode(str) {
        var c1, c2, c3, c4;
        var i, len, out;

        len = str.length;
        i = 0;
        out = "";
        while(i < len) {
            /* c1 */
            do {
                c1 = base64DecodeChars[str.charCodeAt(i++) & 0xff];
            } while(i < len && c1 == -1);
            if(c1 == -1)
                break;

            /* c2 */
            do {
                c2 = base64DecodeChars[str.charCodeAt(i++) & 0xff];
            } while(i < len && c2 == -1);
            if(c2 == -1)
                break;

            out += String.fromCharCode((c1 << 2) | ((c2 & 0x30) >> 4));

            /* c3 */
            do {
                c3 = str.charCodeAt(i++) & 0xff;
                if(c3 == 61)
                    return out;
                c3 = base64DecodeChars[c3];
            } while(i < len && c3 == -1);
            if(c3 == -1)
                break;

            out += String.fromCharCode(((c2 & 0XF) << 4) | ((c3 & 0x3C) >> 2));

            /* c4 */
            do {
                c4 = str.charCodeAt(i++) & 0xff;
                if(c4 == 61)
                    return out;
                c4 = base64DecodeChars[c4];
            } while(i < len && c4 == -1);
            if(c4 == -1)
                break;
            out += String.fromCharCode(((c3 & 0x03) << 6) | c4);
        }
        return out;
    }

    var scope = (typeof window !== "undefined") ? window : self;
    if (!scope.btoa) scope.btoa = base64encode;
    if (!scope.atob) scope.atob = base64decode;

})();

There is now native support for this in Chrome: https://codereview.chromium.org/19749003