Bug 879161 - Fix pdf.js fallout from bug 855613. r=bdahl

browser/extensions/pdfjs/content/PdfJs.jsm

@@ -224,7 +224,7 @@ let PdfJs = {
       }
       let mimeTypes = tag.getMimeTypes();
       return mimeTypes.some(function(mimeType) {
-        return mimeType.type === PDF_CONTENT_TYPE;
+        return mimeType === PDF_CONTENT_TYPE;
       });
     });