VrShell: implement insecure content warning display

chrome/browser/resources/vr_shell/vr_shell_ui.css

 /* Copyright 2016 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 html {
   background-color: rgba(255, 255, 255, 0.2);
   font-size: 20px;
 }
 
 .ui-button {
   background: white;
   border-radius: 6px;
   overflow: hidden;
   position: absolute;
   text-align: center;
   vertical-align: middle;
 }
+
+div.webvr-message-box {
+  align-items: center;
+  background: transparent;
+  display: flex;
+  flex-direction: column;
+  justify-content: center;
+  position: absolute;
+}
+
+#webvr-not-secure-transient {
+  height: 128px;
+  left: 0;
+  top: 0;
+  width: 256px;
+}
+
+#webvr-not-secure-transient > div {
+  background-color: rgba(0, 0, 0, 0.5);
+  border-radius: 6px;
+  color: white;
+  display: flex;
+  flex-direction: column;
+  font-family: sans-serif;
+  font-size: 15px;
+  justify-content: center;
+  max-height: 108px;  /* parent height - 2 * padding */
+  min-height: 80px;
+  padding: 5px;
+  text-align: center;
+  width: 246px;  /* parent width - 2 * padding */
+}
+
+#webvr-not-secure-permanent {
+  height: 64px;
+  left: 256px;
+  top: 0;
+  width: 128px;
+}
+
+#webvr-not-secure-permanent > span {
+  background: white;
+  border-radius: 3px;
+  box-shadow: 0 0 5px rgba(0,0,0,0.3);
+  color: #444;
+  font-family: sans-serif;
+  font-size: 16px;
+  max-width: 112px;  /* parent width - 2 * padding - 2 * shadow radius */
+  padding: 3px;
+  text-align: center;
+  white-space: nowrap;
+}