Only define the `validateFileURL` variable in GENERIC builds
Only define the `validateFileURL` variable in GENERIC builds

There's no point in having this variable defined (implicitly) as `undefined` in e.g. the Firefox PDF Viewer.
By defining it with `var` and using an ESLint ignore, rather than `let`, we can move it into the relevant pre-processor block instead. Note that since the entire viewer-code is placed, by Webpack, in a top-level closure this variable will thus not become globally accessible.

Snuffleupagus/pdf.js,Shoobx/pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js,showpad/mozilla-pdf.js,showpad/mozilla-pdf.js,Shoobx/pdf.js,mozilla/pdf.js,mozilla/pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,Snuffleupagus/pdf.js,showpad/mozilla-pdf.js,Shoobx/pdf.js,timvandermeij/pdf.js