Ignore overhead slices in cpu time computation.

tracing/tracing/metrics/system_health/cpu_time_metric.html

Index: tracing/tracing/metrics/system_health/cpu_time_metric.html
diff --git a/tracing/tracing/metrics/system_health/cpu_time_metric.html b/tracing/tracing/metrics/system_health/cpu_time_metric.html
index eed94f0365adb8168b9e73d6055be8cbc707bb91..caef2cab20ca9af1a6a9647058a667c686d85be6 100644
--- a/tracing/tracing/metrics/system_health/cpu_time_metric.html
+++ b/tracing/tracing/metrics/system_health/cpu_time_metric.html
@@ -25,10 +25,12 @@ tr.exportTo('tr.metrics.sh', function() {
   function cpuTimeMetric(values, model, optOptions) {
     var rangeOfInterest = optOptions ? optOptions.rangeOfInterest : undefined;
     var allProcessCpuTime = 0;
+    var allOverhead = 0;
 
     for (var pid in model.processes) {
       var process = model.processes[pid];
       var processCpuTime = 0;
+      var processOverhead = 0;
       for (var tid in process.threads) {
         var thread = process.threads[tid];
         var threadCpuTime = 0;
@@ -40,10 +42,26 @@ tr.exportTo('tr.metrics.sh', function() {
           threadCpuTime += slice.cpuDuration;
         });
         processCpuTime += threadCpuTime;
+
+        var threadOverhead = 0;
+        thread.sliceGroup.slices.forEach(function(slice) {
+          if (rangeOfInterest &&
+              !rangeOfInterest.intersectsExplicitRangeInclusive(
+                  slice.start, slice.end))
+            return;
+          if (slice.category != 'trace_event_overhead' ||
+              slice.name != 'overhead')
+            return;
+          threadOverhead += slice.cpuDuration;
+        });
+        processOverhead += threadOverhead;
       }
       allProcessCpuTime += processCpuTime;
+      allOverhead += processOverhead;
     }
 
+    allProcessCpuTime -= allOverhead;
+
     // Normalize cpu time by clock time.
     var normalizationRange = rangeOfInterest ?
         rangeOfInterest : model.bounds.range;