Bug 1291709 Make PdfjsChromeUtils.jsm use a WeakSet for tracking browsers. r=bdahl

browser/extensions/pdfjs/content/PdfjsChromeUtils.jsm

@@ -64,7 +64,7 @@ var PdfjsChromeUtils = {
    */
 
   init: function () {
-    this._browsers = new Set();
+    this._browsers = new WeakSet();
     if (!this._ppmm) {
       // global parent process message manager (PPMM)
       this._ppmm = Cc['@mozilla.org/parentprocessmessagemanager;1'].