Devtools: Add editable keyframes to the styles sidebar pane

third_party/WebKit/LayoutTests/inspector/elements/styles/resources/keyframes.css

 #element {
     animation: animName 1s 2s, mediaAnim 2s, doesNotExist 3s, styleSheetAnim 0s;
 }
 
 @keyframes styleSheetAnim {
     from {
-        width: 100px;
+        padding-left: 100px;
     }
     10% {
-        width: 150px;
+        padding-left: 150px;
     }
     100% {
-        width: 200px;
+        padding-left: 200px;
     }
 }
 
 @media (min-width: 1px) {
     @keyframes mediaAnim {
         from {
             opacity: 0;
         }
         to {
             opacity: 1;
         }
     }
 }