[Sync] Start implementation of migration for USS.

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 2b78b57ecd7e2c45a0572fd2689cf6b438bad4fb..e47898e63a7a3bad71bea9875a2bf8351c777b6a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -63892,6 +63892,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Time it took sync to load models for USS datatypes.</summary>
 </histogram>
 
+<histogram name="Sync.USSMigrationFailure" enum="SyncModelTypes">
+  <owner>maxbogue@chromium.org</owner>
+  <summary>Counts directory to USS migration failures per model type.</summary>
+</histogram>
+
+<histogram name="Sync.USSMigrationSuccess" enum="SyncModelTypes">
+  <owner>maxbogue@chromium.org</owner>
+  <summary>Counts directory to USS migration successes per model type.</summary>
+</histogram>
+
 <histogram name="Sync.WifiCredentialsAssociationTime" units="ms">
   <obsolete>
     Deprecated in M53.