Scroll on main if impl-hit testing isn't guaranteed to be correct

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 e172e6bc64845865971ed12a2632bbd4324bf863..63ece9d0db119040fe8f5176798a950ec4b10b8d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -20878,6 +20878,17 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Renderer4.CompositorScrollHitTestResult"
+    enum="CompositorScrollResult">
+  <owner>vollick@chromium.org</owner>
+  <summary>
+    It's possible for compositor hit testing to determine conclusively that
+    compositor thread scrolling can or cannot be done. It's also possible that
+    the hit testing result is inconclusive. We would like to see the I-don't-
+    know result as little as possible. This histogram tracks the ratios.
+  </summary>
+</histogram>
+
 <histogram name="Renderer4.CompositorThreadImplDrawDelay" units="milliseconds">
   <owner>wiltzius@chromium.org</owner>
   <summary>
@@ -31142,6 +31153,13 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="2" label="Will use composited scrolling"/>
 </enum>
 
+<enum name="CompositorScrollResult" type="int">
+  <int value="0" label="ScrollOnMainThread"/>
+  <int value="1" label="ScrollStarted"/>
+  <int value="2" label="ScrollIgnored"/>
+  <int value="3" label="ScrollUnknown"/>
+</enum>
+
 <enum name="CompositorType" type="int">
   <int value="0" label="Software compositor"/>
   <int value="1" label="GPU compositor"/>