Always show the overflow indicator

On the bookmarks toolbar.

Fix #6869.

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

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

Test Plan:

app/common/lib/bookmarkUtil.js

@@ -75,7 +75,7 @@ const getToolbarBookmarks = (state) => {
  .sort(siteUtil.siteSort)
  .filter((bookmark) => !bookmark.get('parentFolderId'))
  let widthAccountedFor = 0
- const overflowButtonWidth = 25
+
  const onlyFavicon = showOnlyFavicon()
  const favicon = showFavicon()
 
@@ -92,7 +92,11 @@ const getToolbarBookmarks = (state) => {
  const fontFamily = domUtil.getStyleConstants('default-font-family')
  const chevronWidth = chevronMargin + fontSize
  const margin = favicon && onlyFavicon ? 0 : bookmarkItemMargin
+
  const windowWidth = window.innerWidth
+ const overflowButtonWidth = 25
+ const maximumBookmarksToolbarWidth = windowWidth - overflowButtonWidth
+
  widthAccountedFor += toolbarPadding
 
  // Loop through until we fill up the entire bookmark toolbar width
@@ -106,15 +110,38 @@ const getToolbarBookmarks = (state) => {
  }
  const currentChevronWidth = favicon && current.get('folderId') ? chevronWidth : 0
  if (favicon && onlyFavicon) {
- widthAccountedFor += padding + iconWidth + currentChevronWidth
+ const extraWidth = padding + iconWidth + currentChevronWidth
+
+ widthAccountedFor += extraWidth
+
+ if (widthAccountedFor >= maximumBookmarksToolbarWidth) {
+ widthAccountedFor -= extraWidth
+ i--
+
+ break
+ }
  } else {
  const text = current.get('customTitle') || current.get('title') || current.get('location')
- widthAccountedFor += Math.min(calculateTextWidth(text, `${fontSize} ${fontFamily}`) + padding + iconWidth + currentChevronWidth, maxWidth)
+ const extraWidth = Math.min(calculateTextWidth(text, `${fontSize} ${fontFamily}`) + padding + iconWidth + currentChevronWidth, maxWidth)
+
+ widthAccountedFor += extraWidth
+
+ if (widthAccountedFor >= maximumBookmarksToolbarWidth) {
+ widthAccountedFor -= extraWidth
+ i--
+
+ break
+ }
  }
  widthAccountedFor += margin
- if (widthAccountedFor >= windowWidth - overflowButtonWidth) {
+
+ if (widthAccountedFor >= maximumBookmarksToolbarWidth) {
+ widthAccountedFor -= margin
+ i--
+
  break
  }
+
  i++
  }