Add a percentage to each metric in the V8 timeline plot

With the percentage, we can easily figure out how much execution time is spent in what metric.

c.f. https://docs.google.com/a/google.com/file/d/0B7GPzKNdEGeQU04ySy1qOWNuLWs/edit

BUG=

Review URL: https://chromiumcodereview.appspot.com/11975045
Patch from Kentaro Hara <haraken@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13415 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

tools/plot-timer-events.js

@@ -502,11 +502,25 @@ function GnuplotOutput() {
   print("set xtics out nomirror");
   print("unset key");
 
+  var percentages = {};
+  var total = 0;
+  for (var name in TimerEvents) {
+    var event = TimerEvents[name];
+    var ranges = MergeRanges(event.ranges);
+    var exclude_ranges = [new Range(-Infinity, xrange_start),
+                          new Range(xrange_end, Infinity)];
+    ranges = ExcludeRanges(ranges, exclude_ranges);
+    var sum =
+      ranges.map(function(range) { return range.duration(); })
+          .reduce(function(a, b) { return a + b; }, 0);
+    percentages[name] = (sum / (xrange_end - xrange_start) * 100).toFixed(1);
+  }
+
   // Name Y-axis.
   var ytics = [];
   for (name in TimerEvents) {
     var index = TimerEvents[name].index;
-    ytics.push('"' + name + '"' + ' ' + index);
+    ytics.push('"' + name + ' (' + percentages[name] + '%%)" ' + index);
   }
   ytics.push('"code kind being executed"' + ' ' + (kY1Offset - 1));
   ytics.push('"top ' + kStackFrames + ' js stack frames"' + ' ' +