DevTools: polish Mac UI (borders, alignment, line heights, images).

Source/devtools/front_end/splitView.css

 /*
  * Copyright (C) 2011 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:
  *
  * 1. Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *
@@ skipping 92 matching lines @@
     top: 4px;
     right:2px;
 }
 
 .split-view button.right-sidebar-show-hide-button:active {
     top: 5px;
     right: 1px;
 }
 
 .split-view button.bottom-sidebar-show-hide-button {
-    bottom: 1px;
+    bottom: 0px;
     right: 1px;
 }
 
 .split-view button.bottom-sidebar-show-hide-button:active {
     bottom: 0;
     right: 0;
 }
 
 .split-view button.left-sidebar-show-hide-button.toggled-show > .glyph {
     -webkit-mask-position: -168px -76px; /* |> */
@@ skipping 23 matching lines @@
 
 .split-view button.bottom-sidebar-show-hide-button.toggled-show > .glyph {
     -webkit-mask-position: -296px -76px; /* <| */
     -webkit-transform: rotate(90deg);
 }
 
 .split-view button.bottom-sidebar-show-hide-button.toggled-hide > .glyph {
     -webkit-mask-position: -264px -76px; /* >| */
     -webkit-transform: rotate(90deg);
 }