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 59793169f1b928a1b3ef9a4a684084bc7654830d..566d700f255ebaa05a91795c374ff2c30aa84fc0 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -16861,6 +16861,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Event.Scroll.ScrollerSize.OnScroll" units="pixels">
+  <owner>yigu@chromium.org</owner>
+  <owner>input-dev@chromium.org</owner>
+  <owner>threaded-rendering@chromium.org</owner>
+  <summary>
+    Record the area of a scroller upon ScrollBegin. This is intended to help us
+    measure the frequencies of different sizes of scrollers getting scrolled.
+    For those small scrollers that users may rarely scroll, there is no need to
+    composite them even if we are able to do so.
+  </summary>
+</histogram>
+
 <histogram name="Event.SingleTapType" enum="TapDelayType">
   <owner>tdresser@chromium.org</owner>
   <owner>input-dev@chromium.org</owner>
@@ -119372,6 +119384,14 @@ from previous Chrome versions.
   <affected-histogram name="Geolocation.SettingsDialog.SuppressEvent"/>
 </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."/>