Add initial UMA metrics for ScrollAnchoring

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 20ee3380fdc1ccab79819fad4fad0f403a7f034e..826ed31c32341dbeee1473cb657017d54302ee47 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -18429,6 +18429,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Layout.ScrollAnchor.AdjustedScrollOffset" enum="BooleanHit">
+  <owner>skobes@chromium.org</owner>
+  <summary>
+    Records the number of times ScrollAnchor adjusts the scroll offset.
+  </summary>
+</histogram>
+
 <histogram name="LevelDB.Open" enum="LevelDBStatus">
   <owner>cmumford@chromium.org</owner>
   <summary>The result of an open attempt of a leveldb.</summary>
@@ -68863,6 +68870,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1272" label="V8Window_SpeechSynthesis_AttributeGetter"/>
   <int value="1273" label="V8IDBFactory_WebkitGetDatabaseNames_Method"/>
   <int value="1274" label="ImageDocument"/>
+  <int value="1275" label="ScrollAnchored"/>
 </enum>
 
 <enum name="FetchRequestMode" type="int">