[DevTools] Remove unnecessary emulation checkboxes and simplify UX.

Source/devtools/front_end/overrides.css

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 194 matching lines @@
 
 .overrides-device, .overrides-media, .overrides-network, .overrides-sensors {
     flex: none !important;
 }
 
 .overrides-sensors input {
     text-align: right;
 }
 
 .overrides-activate-device #tab-device,
-.overrides-activate-media #tab-viewport,
+.overrides-activate-media #tab-media,
 .overrides-activate-network #tab-network,
 .overrides-activate-sensors #tab-sensors {
     opacity: 0.8;
 }
 
 .overrides-activate-device #tab-device .tabbed-pane-header-tab-title::after,
-.overrides-activate-media #tab-viewport .tabbed-pane-header-tab-title::after,
+.overrides-activate-media #tab-media .tabbed-pane-header-tab-title::after,
 .overrides-activate-network #tab-network .tabbed-pane-header-tab-title::after,
 .overrides-activate-sensors #tab-sensors .tabbed-pane-header-tab-title::after {
     padding-left: 3px;
     content: "\2713";
     color: rgb(25, 100, 228);
 }
 
 .overrides-activate-device #tab-device.selected,
-.overrides-activate-media #tab-viewport.selected,
+.overrides-activate-media #tab-media.selected,
 .overrides-activate-network #tab-network.selected,
 .overrides-activate-sensors #tab-sensors.selected {
     opacity: 1;
 }
 
 .overrides-view input[type=text]:enabled:focus,
 .overrides-view select:enabled:focus {
     -webkit-transition: border-color 200ms;
     border-color: rgb(77, 144, 254);
     outline: none;
@@ skipping 41 matching lines @@
 
 @media (-webkit-min-device-pixel-ratio: 1.5) {
 .overrides-view .overrides-footer::before {
     background-image: url(Images/statusbarButtonGlyphs_2x.png);
 }
 } /* media */
 
 .overrides-view .overrides-footer .warning-icon-small {
     margin-right: 3px;
 }