[tools] map-processor updates

- Show map description for selected maps
- More instructions for detailed investigation

Change-Id: Ia07e847c20cafc52160310c7a358004330d22094
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2231356Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68208}

tools/map-processor.html

@@ -1259,7 +1259,8 @@ function transitionTypeToColor(type) {
 
   <section>
     <h2>Instructions</h2>
-    <p>Visualize Map trees that have been gathered using <code>--trace-maps</code>.</p>
+    <p>Visualize Map trees that have been gathered using <code>path/to/d8 $FILE --trace-maps</code>.</p>
+    <p>You can inspect the transition tree in DevTools by looking at <code>document.state.timeline.values</code>.
     <h3>Keyboard Shortcuts</h3>
     <dl>
       <dt><kbd>SHIFT</kbd> + <kbd>Arrow Up</kbd></dt>

tools/map-processor.js

@@ -193,9 +193,7 @@ class MapProcessor extends LogReader {
   processMapDetails(time, id, string) {
     //TODO(cbruni): fix initial map logging.
     let map = this.getExistingMap(id, time);
-    if (!map.description) {
-      //map.description = string;
-    }
+    if (!map.description) map.description = string;
   }
 
   createMap(id, time) {