[WebUI] Toast button shuld not have hairline

the button in toasts of Bookmarks should not have a border.

Bug: 842013
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I2d431ba60b598125730eaf56b618fe3e9b1d9ccd
Reviewed-on: https://chromium-review.googlesource.com/1055132
Commit-Queue: Hwanseung Lee <hwanseung@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558200}

chrome/browser/resources/md_bookmarks/toast_manager.html

@@ -17,6 +17,7 @@
       paper-button {
         -webkit-margin-end: 0;
         -webkit-margin-start: 32px;
+        border: none;
         color: var(--google-blue-300);
         height: 32px;
         padding: 8px;