cc: Fixed computation of dropped frames.

- Computation of dropped frames now correctly accounts for multi-frame drops.
- A welcome side-effect is that variance of the percentage_dropped metric
  is reduced significantly.
- Heads-up for the perf sherrifs: percentage_dropped will increase quite a bit
  with this patch.

R=tonyg@chromium.org, nduca@chromium.org
BUG=264308

Review URL: https://chromiumcodereview.appspot.com/23998002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221825 0039d316-1c4b-4281-b951-d872f2087c98

cc/debug/frame_rate_counter.cc

@@ -73,7 +73,8 @@ void FrameRateCounter::SaveTimeStamp(base::TimeTicks timestamp, bool software) {
 
   if (!IsBadFrameInterval(frame_interval_seconds) &&
       frame_interval_seconds.InSecondsF() > kDroppedFrameTime)
-    ++dropped_frame_count_;
+    dropped_frame_count_ +=
+        frame_interval_seconds.InSecondsF() / kDroppedFrameTime;
 }
 
 bool FrameRateCounter::IsBadFrameInterval(

tools/perf/metrics/smoothness.js

@@ -149,10 +149,11 @@
 
   RafRenderingStats.prototype.getDroppedFrameCount_ = function(frameTimes) {
     var droppedFrameCount = 0;
+    var droppedFrameThreshold = 1000 / 55;
     for (var i = 1; i < frameTimes.length; i++) {
       var frameTime = frameTimes[i] - frameTimes[i-1];
-      if (frameTime > 1000 / 55)
-        droppedFrameCount++;
+      if (frameTime > droppedFrameThreshold)
+        droppedFrameCount += Math.floor(frameTime / droppedFrameThreshold);
     }
     return droppedFrameCount;
   };

tools/perf/metrics/smoothness.py

@@ -157,8 +157,8 @@ def CalcResults(benchmark_stats, results):
               round(Median(frame_times), 2))
 
   results.Add('dropped_percent', '%',
-              Average(s.dropped_frame_count, s.screen_frame_count,
-                      100, 1),
+              Average(s.dropped_frame_count,
+                      s.screen_frame_count + s.dropped_frame_count, 100, 1),
               data_type='unimportant')
   results.Add('percent_impl_scrolled', '%',
               Average(s.impl_thread_scroll_count,