[Sync] Record histogram of ModelTypeStoreBackend initialization result

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 ad9ed02810347e0cda79a24caaa94a11a526c90f..587677410c4b5af26ea30ccbbc2807949e34547b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -67500,6 +67500,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Sync.ModelTypeStoreInitResult"
+    enum="SyncModelTypeStoreInitResult">
+  <owner>pavely@chromium.org</owner>
+  <summary>
+    Records the result of ModelTypeStoreBackend initialization. Used to analyze
+    frequency and causes of backend initialization failures.
+  </summary>
+</histogram>
+
 <histogram name="Sync.NigoriMigrationState" enum="SyncNigoriMigrationState">
   <owner>zea@chromium.org</owner>
   <summary>Breakdown of sync's nigori node keystore migration state.</summary>
@@ -107441,6 +107450,18 @@ value.
   <int value="38" label="Reading List"/>
 </enum>
 
+<enum name="SyncModelTypeStoreInitResult" type="int">
+  <int value="0" label="Success"/>
+  <int value="1" label="Not found"/>
+  <int value="2" label="Corruption"/>
+  <int value="3" label="Not supported"/>
+  <int value="4" label="Invalid argument"/>
+  <int value="5" label="IO error"/>
+  <int value="6" label="Schema descriptor issue"/>
+  <int value="7" label="Migration failed"/>
+  <int value="8" label="Unknown error"/>
+</enum>
+
 <enum name="SyncNigoriMigrationResult" type="int">
   <int value="0" label="Failed to set default encryption key"/>
   <int value="1" label="Failed to set nondefault encryption key"/>