Record size of scroller that user scrolls

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index b9111bb3ddfc3f3016e3dcb3737f9482bcee38ae..ae3ec07939226008f1077fac6914294443545a07 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -16592,6 +16592,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Event.Scroll.ScrollerSize.OnScroll" units="squared pixels">

[tdresser, 2017/03/28 13:30:12]

The only histogram using "squared pixels" for the units is actually reporting
the distance between two things in pixels, squared.

That's pretty different from this.

I think just saying this is in pixels would probably be reasonable.

[flackr, 2017/03/28 14:40:23]

To be clear, you mean saying the units are pixels but still measuring the area
right? The nice thing about area is it is linearly correlated with GPU memory
usage.

+  <owner>yigu@chromium.org</owner>
+  <owner>input-dev@chromium.org</owner>
+  <owner>threaded-rendering@chromium.org</owner>
+  <summary>
+    Record the size of a scroller upon ScrollBegin. This is intended to help us
+    measuring the frequencies of different sizes of scrollers getting scrolled.

[tdresser, 2017/03/28 13:30:12]

measuring -> measure

[yigu, 2017/03/29 15:32:03]

Done.

+    For those small scrollers that users may rarely scroll, there is no need to
+    composite them even we are able to do so.

[tdresser, 2017/03/28 13:30:12]

even -> even if

[yigu, 2017/03/29 15:32:03]

Done.

+  </summary>
+</histogram>
+
 <histogram name="Event.SingleTapType" enum="TapDelayType">
   <owner>tdresser@chromium.org</owner>
   <owner>input-dev@chromium.org</owner>
@@ -117663,6 +117675,14 @@ from previous Chrome versions.
   <affected-histogram name="PLT.PT_StartToFinish"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="Gesture">
+  <suffix name="Wheel"
+      label="Measure the size of scroller that users wheel scroll"/>
+  <suffix name="Touch"
+      label="Measure the size of scroller that users touch scroll"/>
+  <affected-histogram name="Event.Scroll.ScrollerSize.OnScroll"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="GLApisWithErrorReporting">
   <suffix name="TexImage2D" label="All GL APIs that allocate a 2D texture."/>
   <suffix name="TexImage3D" label="All GL APIs that allocate a 3D texture."/>