Add scroll prediction over/under predict metric

This CL adds two metrics for scroll event prediction to help analyse
prediction behavior, Scroll.OverPredict and Scroll.UnderPredict.

The new metrics only records the y direction difference of prediction
result instead of euclidean distance, since y direction scroll is more
popular and only record one direction is easier for determine over/under
predict.


Bug: 836352
Change-Id: I2f83fa5404b02c52f8856dfb64fa0151b7b686d6
Reviewed-on: https://chromium-review.googlesource.com/c/1354314Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Commit-Queue: Ella Ge <eirage@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612401}

tools/metrics/histograms/histograms.xml

@@ -27164,6 +27164,41 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="Event.InputEventPrediction.Accuracy.Scroll.OverPredict"
+    units="pixels" expires_after="M80">
+<!-- Name completed by histogram_suffixes
+            name="InputEventPredictionAccuracy" -->
+
+  <owner>eirage@chromium.org</owner>
+  <summary>
+    Records y direction distance from a ScrollUpdate event's absolute scroll
+    position (count by accumulated delta) to the predicted scrolling position at
+    the event time when prediction is available and also the predicted result is
+    'OverPredict'. Over predict means the absolute value of predicted scroll
+    delta is larger than the real delta in the same time period.
+
+    Team: input-dev@chromium.org.
+  </summary>
+</histogram>
+
+<histogram name="Event.InputEventPrediction.Accuracy.Scroll.UnderPredict"
+    units="pixels" expires_after="M80">
+<!-- Name completed by histogram_suffixes
+            name="InputEventPredictionAccuracy" -->
+
+  <owner>eirage@chromium.org</owner>
+  <summary>
+    Records y direction distance from a ScrollUpdate event's absolute scroll
+    position (count by accumulated delta) to the predicted scrolling position at
+    the event time when prediction is available and also the predicted result is
+    'UnderPredict'. Under predict means the absolute value of predicted scroll
+    delta is smaller than the real delta in the same time period.
+    GestureScrollUpdate events.
+
+    Team: input-dev@chromium.org.
+  </summary>
+</histogram>
+
 <histogram name="Event.InputEventPrediction.Accuracy.Touch" units="pixels">
 <!-- Name completed by histogram_suffixes
             name="InputEventPredictionAccuracy" -->
@@ -131052,6 +131087,10 @@ uploading your change for review.
   <suffix name="Short" label="predicted time less than 10ms."/>
   <affected-histogram name="Event.InputEventPrediction.Accuracy.Mouse"/>
   <affected-histogram name="Event.InputEventPrediction.Accuracy.Scroll"/>
+  <affected-histogram
+      name="Event.InputEventPrediction.Accuracy.Scroll.OverPredict"/>
+  <affected-histogram
+      name="Event.InputEventPrediction.Accuracy.Scroll.UnderPredict"/>
   <affected-histogram name="Event.InputEventPrediction.Accuracy.Touch"/>
 </histogram_suffixes>
 

ui/events/blink/scroll_predictor.cc

@@ -175,6 +175,20 @@ void ScrollPredictor::ComputeAccuracy(const WebScopedInputEvent& event) {
     base::UmaHistogramCounts1000(
         "Event.InputEventPrediction.Accuracy.Scroll." + suffix,
         static_cast<int>(distance));
+
+    // If the distance from predicted position to actual position is in same
+    // direction as the delta_y, the result is under predicted, otherwise over
+    // predict.
+    float dist_y = temporary_accumulated_delta_.y() - predict_result.pos.y();
+    if (gesture_event.data.scroll_update.delta_y * dist_y < 0) {
+      base::UmaHistogramCounts1000(
+          "Event.InputEventPrediction.Accuracy.Scroll.OverPredict." + suffix,
+          static_cast<int>(std::abs(dist_y)));
+    } else {
+      base::UmaHistogramCounts1000(
+          "Event.InputEventPrediction.Accuracy.Scroll.UnderPredict." + suffix,
+          static_cast<int>(std::abs(dist_y)));
+    }
   }
 }