DevTools: Show asynchronous call stacks on frontend.

Source/devtools/front_end/inspector.css

Index: Source/devtools/front_end/inspector.css
diff --git a/Source/devtools/front_end/inspector.css b/Source/devtools/front_end/inspector.css
index 6886b557198ad72667e8c7f2d76370f198b94cc6..b29a007a085c24ef289ef3bd6d1afda17d996749 100644
--- a/Source/devtools/front_end/inspector.css
+++ b/Source/devtools/front_end/inspector.css
@@ -1405,11 +1405,16 @@ iframe.panel.extension {
 
 .placard {
     position: relative;
+    border-top: 1px solid transparent;
     padding: 3px 8px 4px 18px;
     min-height: 18px;
     white-space: nowrap;
 }
 
+.placard.grouped {
+    padding-left: 36px;
+}
+
 .placard:nth-of-type(2n) {
     background-color: rgb(234, 243, 255);
 }