Re-landing "UMA metrics for fractions of wheel and touch scrolls blocked on the main thread."

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 55bd86afe56caf591df53ac0227b5480823cf472..702fdae41e6bfdab4e286220df9fc8efe5f6ff73 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -53417,6 +53417,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Renderer4.GestureScrollingThreadStatus"
+    enum="ScrollingThreadStatus">
+  <owner>tdresser@chromium.org</owner>
+  <summary>
+    For every gesture, we record whether the scroll occurred on the main thread,
+    on the compositor thread, or on the compositor thread but blocked on the
+    main thread. The last case will happen when there is a blocking event
+    listener.
+  </summary>
+</histogram>
+
 <histogram name="Renderer4.GpuImageDecodeState" enum="GpuImageDecodeState">
   <owner>vmpstr@chromium.org</owner>
   <summary>
@@ -53815,6 +53826,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Renderer4.WheelScrollingThreadStatus"
+    enum="ScrollingThreadStatus">
+  <owner>tdresser@chromium.org</owner>
+  <summary>
+    For every wheel tick, we record whether the scroll occurred on the main
+    thread, on the compositor thread, or on the compositor thread but blocked on
+    the main thread. The last case will happen when there is a blocking event
+    listener.
+  </summary>
+</histogram>
+
 <histogram name="RendererScheduler.BackgroundRendererMainThreadLoad" units="%">
   <owner>altimin@chromium.org</owner>
   <summary>
@@ -104476,6 +104498,12 @@ value.
   <int value="9" label="Rollback: SBER2 absent so SBER1 must be cleared"/>
 </enum>
 
+<enum name="ScrollingThreadStatus" type="int">
+  <int value="0" label="SCROLLING_ON_COMPOSITOR"/>
+  <int value="1" label="SCROLLING_ON_COMPOSITOR_BLOCKED_ON_MAIN"/>
+  <int value="2" label="SCROLLING_ON_MAIN"/>
+</enum>
+
 <enum name="ScrollThread" type="int">
   <int value="0" label="Scroll on impl-thread"/>
   <int value="1" label="Scroll on main-thread"/>