DevTools: add emulation toggle button and splash screen.

Source/devtools/front_end/elements/module.json

 {
     "extensions": [
         {
             "type": "@WebInspector.Panel",
             "name": "elements",
             "title": "Elements",
             "order": 0,
             "className": "WebInspector.ElementsPanel"
         },
         {
@@ skipping 24 matching lines @@
             "type": "@WebInspector.Revealer",
             "contextTypes": ["WebInspector.DOMNode"],
             "className": "WebInspector.ElementsPanel.DOMNodeRevealer"
         },
         {
             "type": "@WebInspector.Revealer",
             "contextTypes": ["WebInspector.RemoteObject"],
             "className": "WebInspector.ElementsPanel.NodeRemoteObjectRevealer"
         },
         {
+            "type": "@WebInspector.Revealer",
+            "contextTypes": ["WebInspector.OverridesSupport"],
+            "className": "WebInspector.OverridesView.Revealer"
+        },
+        {
             "type": "ui-setting",
             "section": "Elements",
             "title": "Color format",
             "settingName": "colorFormat",
             "settingType": "select",
             "options": [
                 [ "As authored", "original" ],
                 [ "HEX: #DAC0DE", "hex", true ],
                 [ "RGB: rgb(128, 255, 255)", "rgb", true ],
                 [ "HSL: hsl(300, 80%, 90%)", "hsl", true ]
@@ skipping 23 matching lines @@
         {
             "type": "ui-setting",
             "section": "Elements",
             "title": "Show rulers",
             "settingName": "showMetricsRulers",
             "settingType": "checkbox"
         }
     ],
     "scripts": [ "ElementsPanel.js" ]
 }