Always show the overflow indicator

On the bookmarks toolbar.

Fix #6869.

However, there is too much space before the overflow indicator.

I'm not well versed with CSS so I'd appreciate some pointers.
Auditors:

Test Plan:

app/renderer/components/bookmarks/bookmarksToolbar.js

@@ -143,12 +143,30 @@ class BookmarksToolbar extends ImmutableComponent {
       const chevronWidth = props.showFavicon && noParentItems.getIn([i, 'folderId']) ? this.chevronWidth : 0
       if (props.showFavicon && props.showOnlyFavicon) {
         widthAccountedFor += this.padding + iconWidth + chevronWidth
+
+        if (widthAccountedFor >= props.windowWidth - overflowButtonWidth) {
+          widthAccountedFor -= this.padding + iconWidth + chevronWidth
+          i--
+
+          break
+        }
       } else {
         const text = noParentItems.getIn([i, 'customTitle']) || noParentItems.getIn([i, 'title']) || noParentItems.getIn([i, 'location'])
         widthAccountedFor += Math.min(calculateTextWidth(text, `${this.fontSize} ${this.fontFamily}`) + this.padding + iconWidth + chevronWidth, this.maxWidth)
+
+        if (widthAccountedFor >= props.windowWidth - overflowButtonWidth) {
+          widthAccountedFor -= Math.min(calculateTextWidth(text, `${this.fontSize} ${this.fontFamily}`) + this.padding + iconWidth + chevronWidth, this.maxWidth)
+          i--
+
+          break
+        }
       }
       widthAccountedFor += margin
+
       if (widthAccountedFor >= props.windowWidth - overflowButtonWidth) {
+        widthAccountedFor -= margin
+        i--
+
         break
       }
     }