DevTools: [timeline tree view] render visual of percent in the background.

Source/devtools/front_end/timeline/timelinePanel.css

Index: Source/devtools/front_end/timeline/timelinePanel.css
diff --git a/Source/devtools/front_end/timeline/timelinePanel.css b/Source/devtools/front_end/timeline/timelinePanel.css
index 0cc54806f81b7fede56ebdf31d47a5bcc3cb8f39..186c30d6a995b3ea41e00ac9516912208a6006ba 100644
--- a/Source/devtools/front_end/timeline/timelinePanel.css
+++ b/Source/devtools/front_end/timeline/timelinePanel.css
@@ -1007,6 +1007,7 @@
 
 .timeline-tree-view .data-grid td.numeric-column {
     text-align: right;
+    position: relative;
 }
 
 .timeline-tree-view .data-grid div.profile-multiple-values {
@@ -1014,7 +1015,7 @@
 }
 
 .timeline-tree-view .data-grid span.percent-column {
-    color: #999;
+    color: #888;
     width: 48px;
     display: inline-block;
 }
@@ -1045,3 +1046,17 @@
     overflow: hidden;
     text-overflow: ellipsis;
 }
+
+.timeline-tree-view .data-grid .background-bar {
+    height: 15px;
+    background-color: hsla(0, 100%, 74%, 0.1);
+    position: absolute;
+    right: -2px;
+    bottom: 0px;
+    border-bottom: 1px solid hsl(0, 100%, 74%);
+}
+
+.timeline-tree-view .data-grid .selected .background-bar {
+    background-color: rgba(255,255,255,0.25);
+    border-bottom-color: transparent;
+}