[Histograms] Only add AggregatableHistogram samples if non-zero.

AggregatableHistogramTimer will always add a sample when it is destroyed,
even if there were no AggregatedHistogramTimerScope called within it. This
makes the V8.CompileLazyMicroSeconds histogram not particularly useful since
it is 90% filled with zero entries where v8 execute didn't require any
lazy function compilation

Change-Id: Ia75c8596237b22528cbba3e8ae2b67e28ea54097
Reviewed-on: https://chromium-review.googlesource.com/793452Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49674}

src/counters.h

@@ -404,7 +404,13 @@ class AggregatableHistogramTimer : public Histogram {
  public:
   // Start/stop the "outer" scope.
   void Start() { time_ = base::TimeDelta(); }
-  void Stop() { AddSample(static_cast<int>(time_.InMicroseconds())); }
+  void Stop() {
+    if (time_ != base::TimeDelta()) {
+      // Only add non-zero samples, since zero samples represent situations
+      // where there were no aggregated samples added.
+      AddSample(static_cast<int>(time_.InMicroseconds()));
+    }
+  }
 
   // Add a time value ("inner" scope).
   void Add(base::TimeDelta other) { time_ += other; }