Fix: define check

[tonyjin]

Global define check was causing errors, adding a window.define check and also fixing document viewer to correctly rely on minified pdf.js.

[boxcla]

Verified that @tonyjin has signed the CLA. Thanks for the pull request!

[jeremypress]

This doesn't break a unit test?

[tonyjin]

@jeremypress it did, but in an odd way. I'm not exactly clear why, but this worked:

const amdPresent = !!window.define && typeof define === 'function' && !!define.amd;
const defineRef = amdPresent ? define : undefined;

if (disableAMD && amdPresent) {
    define = undefined;
}

and this didn't:

const amdPresent = !!window.define && typeof define === 'function' && !!define.amd;
let defineRef;

if (disableAMD && amdPresent) {
    defineRef = define;
    define = undefined;
}

Something to do with hoisting?