[DevTools] Hide the device frame when the viewport is too narrow to accomodate it.

third_party/WebKit/Source/devtools/front_end/emulation/deviceModeView.css

 /*
  * Copyright 2015 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.
  */
 
 :host {
     overflow: hidden;
     align-items: stretch;
     flex: auto;
@@ skipping 111 matching lines @@
 }
 
 .device-mode-screen-image {
     position: absolute;
     left: 0;
     top: 0;
     width: 100%;
     height: 100%;
 }
 
+.device-mode-outline-image {
+  visibility: hidden;
+}
+
+.device-mode-outline-image.device-frame-visible {
+  visibility: visible;
+}
+
 .device-mode-outline-image.device-frame-visible + .device-mode-screen-area {
     box-shadow: none;
 }
 
 .device-mode-resizer {
     position: absolute;
     display: flex;
     align-items: center;
     justify-content: center;
     overflow: hidden;
@@ skipping 185 matching lines @@
 .device-mode-ruler-top .device-mode-ruler-text {
     left: 2px;
     top: -2px;
 }
 
 .device-mode-ruler-left .device-mode-ruler-text {
     left: -4px;
     top: -15px;
     transform: rotate(270deg);
 }
+
+@media (max-width : 520px) {
+  .device-mode-outline-image.device-frame-visible {
+    visibility: hidden;
+  }
+
+  .device-mode-outline-image.device-frame-visible + .device-mode-screen-area {
+    box-shadow: hsl(240, 3%, 84%) 0 0 0 0.5px, hsla(0, 0%, 80%, 0.4) 0 0 20px;
+  }
+}