Track async causal stack traces

runtime/observatory/lib/src/elements/css/shared.css

 /* Global styles */
 * {
   margin: 0;
   padding: 0;
   box-sizing: border-box;
 }
 
 body {
   padding-top: 56px;
   color: #333;
@@ skipping 881 matching lines @@
   padding: 5px;
   border: 1px solid white;
 }
 debugger-frame:hover {
   border: 1px solid #e0e0e0;
 }
 debugger-frame.shadow {
   box-shadow:  0 2px 10px 0 rgba(0, 0, 0, 0.16),
                0 2px 5px 0 rgba(0, 0, 0, 0.26);
 }
+debugger-frame.causalFrame {
+  background-color: #D7CCC8;
+}
 debugger-frame.current {
   box-shadow:  0 2px 10px 0 rgba(0, 0, 0, 0.26),
                0 2px 5px 0 rgba(0, 0, 0, 0.46);
   border: 1px solid #444;
 }
 debugger-frame > button {
   display: block;
   width: 100%;
   text-align: left;
   background-color: transparent;
@@ skipping 1390 matching lines @@
 
 vm-connect-target > button.delete-button:hover {
   background: #ff0000;
 }
 
 /* vm-connect */
 
 vm-connect ul {
   list-style-type: none;
 }