Bookmarks WebUI: Prevent indicator from shrinking due to large fonts

Bug: 1162022
Change-Id: I7d6d3e5691a52505306a600738b813cc27bc73ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601269Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839621}

ui/webui/resources/cr_elements/shared_style_css.html

@@ -168,6 +168,7 @@
         border-radius: 0 3px 3px 0;
         content: '';
         display: block;
+        flex-shrink: 0;
         height: var(--cr-vertical-tab-height, 100%);
         width: 4px;
       }