Enable the no-else-return ESLint rule

Using `else` after `return` is not necessary, and can often lead to unnecessarily cluttered code. By using the `no-else-return` rule in ESLint we can avoid this pattern, see http://eslint.org/docs/rules/no-else-return.

web/pdf_page_view.js

@@ -624,7 +624,7 @@ var PDFPageView = (function PDFPageViewClosure() {
           onRenderContinue: function (cont) { },
           cancel: function () { },
         };
-      } else {
+      } else { // eslint-disable-line no-else-return
         var cancelled = false;
         var ensureNotCancelled = function () {
           if (cancelled) {