about:tracing support for TRACE_ASYNC_START/FINISH events.

chrome/browser/resources/tracing/timeline_track.js

Index: chrome/browser/resources/tracing/timeline_track.js
diff --git a/chrome/browser/resources/tracing/timeline_track.js b/chrome/browser/resources/tracing/timeline_track.js
index 0d00b1e822c3630c3bd8a521c46e2fad0c9fe086..dd40608c195b5cc21f3c563200b67157f1d30acd 100644
--- a/chrome/browser/resources/tracing/timeline_track.js
+++ b/chrome/browser/resources/tracing/timeline_track.js
@@ -181,12 +181,18 @@ cr.define('tracing', function() {
           track.height = '4px';
         }
 
-        for (var srI = 0; srI < this.thread_.nonNestedSubRows.length; ++srI) {
-          this.addTrack_(this.thread_.nonNestedSubRows[srI]);
+        if (this.thread_.asyncSlices.length) {
+          var subRows = this.thread_.asyncSlices.subRows;
+          for (var srI = 0; srI < subRows.length; srI++) {
+            var track = this.addTrack_(subRows[srI]);
+            track.asyncStyle = true;
+          }
         }
-        for (var srI = 0; srI < this.thread_.subRows.length; ++srI) {
+
+        for (var srI = 0; srI < this.thread_.subRows.length; srI++) {
           this.addTrack_(this.thread_.subRows[srI]);
         }
+
         if (this.tracks_.length > 0) {
           if (this.thread_.cpuSlices) {
             this.tracks_[1].heading = this.heading_;
@@ -443,6 +449,16 @@ cr.define('tracing', function() {
     decorate: function() {
       this.classList.add('timeline-slice-track');
       this.elidedTitleCache = new ElidedTitleCache();
+      this.asyncStyle_ = false;
+    },
+
+    get asyncStyle() {
+      return this.asyncStyle_;
+    },
+
+    set asyncStyle(v) {
+      this.asyncStyle_ = !!v;
+      this.invalidate();
     },
 
     get slices() {
@@ -503,6 +519,8 @@ cr.define('tracing', function() {
       vp.applyTransformToCanavs(ctx);
 
       // Slices.
+      if (this.asyncStyle_)
+        ctx.globalAlpha = 0.25;
       var tr = new tracing.FastRectRenderer(ctx, viewLWorld, 2 * pixWidth,
                                             2 * pixWidth, viewRWorld, pallette);
       tr.setYandH(0, canvasH);