cc: Fixed computation of dropped frames.

tools/perf/metrics/smoothness.js

Index: tools/perf/metrics/smoothness.js
diff --git a/tools/perf/metrics/smoothness.js b/tools/perf/metrics/smoothness.js
index e563dcb51a48528e508cc381d6dd92c4b76cc331..1fdf975b681a38d3a06f60b7f84c51d779b32925 100644
--- a/tools/perf/metrics/smoothness.js
+++ b/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;
   };