addon: isWindowPrivate -> isContentWindowPrivate

As of Firefox 35, isContentWindowPrivate should be used for DOM windows instead of isWindowPrivate.
See https://bugzilla.mozilla.org/show_bug.cgi?id=1069059 and
http://hg.mozilla.org/mozilla-central/diff/324798b60ba3/toolkit/modules/PrivateBrowsingUtils.jsm

Without this fix, you will get the following error message when Firefox+PDF.js is started:
>
WARNING: content window passed to PrivateBrowsingUtils.isWindowPrivate. Use isContentWindowPrivate instead (but only for frame scripts).
pbu_isWindowPrivate@resource://gre/modules/PrivateBrowsingUtils.jsm:25:14
ChromeActions.prototype.isInPrivateBrowsing@resource://pdf.js/PdfStreamConverter.jsm:237:12
xhr_onreadystatechange@resource://pdf.js/PdfStreamConverter.jsm:545:30
NetworkManager_requestRange@resource://pdf.js/network.js:95:7
NetworkManager_requestRange@resource://pdf.js/network.js:81:14
RangedChromeActions_requestDataRange@resource://pdf.js/PdfStreamConverter.jsm:596:1
RequestListener.prototype.receive@resource://pdf.js/PdfStreamConverter.jsm:705:5
PdfStreamConverter.prototype.onStartRequest/proxy.onStopRequest/<@resource://pdf.js/PdfStreamConverter.jsm:909:11
FirefoxComClosure/<.request@resource://pdf.js/web/viewer.js:529:14
PdfDataRangeTransport_requestDataRange@resource://pdf.js/web/viewer.js:2977:9
transportDataRange@resource://pdf.js/build/pdf.js:2122:13
messageHandlerComObjOnMessage@resource://pdf.js/build/pdf.js:1219:9
Merge pull request #5364 from Rob--W/firefox-pbu_isWindowPrivate

Firefox addon: Use isContentWindowPrivate instead of isWindowPrivate if available