WebUI: Prevent dragging from the logo on user manager page

Bug: 910026
Change-Id: Ifdf6cb51032dd8885297bf3d8d4febaa2e564647
Reviewed-on: https://chromium-review.googlesource.com/c/1357459Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613214}

chrome/browser/resources/md_user_manager/control_bar.html

@@ -21,6 +21,7 @@
 
       #logo {
         margin-inline-end: auto;
+        user-select: none;
       }
 
       paper-button + paper-button {