Refactor the `selectScaleOption` function, in `Toolbar._updateUIState`, to prevent any possible future display glitches

Since the localization service is now asynchronous, depending on the load the browser is under, there's a small risk that the lookup of the 'page_scale_percent' string could be delayed slightly.
If the scale would change a couple of times in *very* quick succession, there's perhaps a *theoretical* possibility that the Zoom dropdown would display an incorrect value.

Consider the following, somewhat contrived, theoretical example of two zoom commands being executed *right* after one another:
```javascript
PDFViewerApplication.pdfViewer.currentScale = 1.23;
PDFViewerApplication.pdfViewer.currentScaleValue = 'page-width';
```

Only the `currentScale` call will currently trigger a l10n lookup in `selectScaleOption`. However, as far as I understand, there's no *guarantee* that the l10n string is resolved *before* `selectScaleOption` is called again as a result of the `currentScaleValue` call.

This thus has the possibility of putting the Zoom dropdown into an inconsistent state, since it's currently updated synchronously for one code-path and asynchronously for another.

To avoid these issues, I'm proposing that we *always* update the Zoom dropdown asynchronously, such that we can guarantee that the ordering is correct.

web/toolbar.js

@@ -180,10 +180,13 @@ var Toolbar = (function ToolbarClosure() {
       }
 
       let selectScaleOption = (value, scale) => {
-        var options = items.scaleSelect.options;
+        let customScale = Math.round(scale * 10000) / 100;
-        var predefinedValueFound = false;
+        this.l10n.get('page_scale_percent', { scale: customScale, },
-        for (var i = 0, ii = options.length; i < ii; i++) {
+                      '{{scale}}%').then((msg) => {
-          var option = options[i];
+          let options = items.scaleSelect.options;
+          let predefinedValueFound = false;
+          for (let i = 0, ii = options.length; i < ii; i++) {
+            let option = options[i];
             if (option.value !== value) {
               option.selected = false;
               continue;
@@ -192,13 +195,10 @@ var Toolbar = (function ToolbarClosure() {
             predefinedValueFound = true;
           }
           if (!predefinedValueFound) {
-          var customScale = Math.round(scale * 10000) / 100;
-          this.l10n.get('page_scale_percent', { scale: customScale, },
-                        '{{scale}}%').then((msg) => {
             items.customScaleOption.textContent = msg;
             items.customScaleOption.selected = true;
-          });
           }
+        });
       };
 
       var pageNumber = this.pageNumber;