Remove scrollbar from the Add Supervision dialog

Bug: 927957
Change-Id: I8624e0f1b0c3ff2d7479e6912fc11dbbfa14b5e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1646999Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Felipe Cerqueira <cerqueira@google.com>
Cr-Commit-Position: refs/heads/master@{#667093}

chrome/browser/resources/chromeos/add_supervision/add_supervision.html

@@ -17,6 +17,8 @@
       html,
       body {
         height: 100%;
+        overflow: auto;
+        overflow-y: hidden;
       }
     </style>
   </head>