DevTools: add emulation toggle button and splash screen.

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 173 matching lines @@
     border-top: 1px solid #EEEEEE;
     margin: 0;
     padding: 12px;
 }
 
 .overrides-view label {
     display: flex;
     height: auto;
 }
 
-/* Screen tab */
+#metrics-override-section > span {
+    padding: 0 10px 0 3px;
+}
+
+#metrics-override-section > select {
+    width: 244px;
+}
+
+/* Media tab */
 
 .overrides-media > label {
     margin-bottom: 4px;
 }
 
 /* Sensors tab */
 
 .overrides-sensors > label {
     margin-bottom: 10px;
 }
@@ skipping 78 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;
 }
+
+.overrides-splash-screen {
+    display: flex;
+    align-items: center;
+    padding: 3px 10px;
+}
+
+.overrides-reset-button {
+    margin: 9px 0 0 17px;
+}