Add metric to know how user use scrollbar

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 b42135ab4be8d99939fdea5df70e578b95f3b3bb..3df0a7342191550ccd3551780a645e66ed2f734f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -57610,6 +57610,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Scrollbar.PressedPart" enum="ScrollbarPressedPart">
+  <owner>chaopeng@chromium.org</owner>
+  <summary>
+    this is about how many users actually use scrollbar (thumb dragging/arrow
+    buttons) to scroll the page. This would be helpful when making decisions
+    related to how overlay scrollbars should work.
+  </summary>
+</histogram>
+
 <histogram name="Sdch3.AdvertisedWithSecureScheme" enum="BooleanHttps">
   <obsolete>
     Experiment complete, histogram gathering code removed.
@@ -103312,6 +103321,15 @@ value.
   <int value="9" label="Rollback: SBER2 absent so SBER1 must be cleared"/>
 </enum>
 
+<enum name="ScrollbarPressedPart" type="int">
+  <int value="1" label="VerticalScrollbarButton"/>
+  <int value="2" label="HorizontalScrollbarButton"/>
+  <int value="3" label="VerticalScrollbarThumb"/>
+  <int value="4" label="HorizontalScrollbarThumb"/>
+  <int value="5" label="VerticalScrollbarTrack"/>
+  <int value="6" label="HorizontalScrollbarTrack"/>
+</enum>
+
 <enum name="ScrollThread" type="int">
   <int value="0" label="Scroll on impl-thread"/>
   <int value="1" label="Scroll on main-thread"/>