MD Extensions: adding border to dev toolbar

Bug: 842099
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ic52a5afc94ab2de1e5745df995441b1c5e6f152b
Reviewed-on: https://chromium-review.googlesource.com/1055815Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558069}

chrome/browser/resources/md_extensions/toolbar.html

@@ -47,6 +47,7 @@
 
       #devDrawer {
         background: white;
+        border-bottom: 1px solid var(--google-grey-300);
         height: 0;
         overflow: hidden;
         position: relative;