Add better consistency check and error output to plot script.

tools/profviz/profviz.html

Index: tools/profviz/profviz.html
diff --git a/tools/profviz/profviz.html b/tools/profviz/profviz.html
index 30494f80fe1ab11f52dc455830c9a22e877be9bb..d7dd9cb95080190fcb107436e907a62ebc1dec58 100644
--- a/tools/profviz/profviz.html
+++ b/tools/profviz/profviz.html
@@ -96,7 +96,7 @@ your computer's performance.">
         or alternatively,<br/>
         Chrome with
         <span class="tt">
-          --no-sandbox --js-flags="--prof --noprof-lazy --log-timer-events
+          --no-sandbox --js-flags="--prof --log-timer-events"
         </span> to produce <span class="tt">v8.log</span>.
       </li>
       <li>
@@ -117,6 +117,11 @@ your computer's performance.">
           command-line utility
         </a> instead.
       </li>
+    </ol>
+    If you expect multiple V8 instances to run concurrently, for example
+    with several tabs in Chrome,<br/>
+    add the V8 flag <span class="tt">--logfile=v8.%p.log</span>
+    so that each instance writes to its own log file.
     </div>
   </div>