Add the Sync.Attachments.StoreInitResult histogram.

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 84859fed502f94effaf0f3cf20a853d5d8b72226..b3b5fe97af2563008bc7bc81042fdbe669179894 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -35924,6 +35924,12 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>Response or error codes from downloading sync attachments.</summary>
 </histogram>
 
+<histogram name="Sync.Attachments.StoreInitResult"
+    enum="SyncAttachmentStoreResult">
+  <owner>maxbogue@chromium.org</owner>
+  <summary>The result of initializing the sync attachment store.</summary>
+</histogram>
+
 <histogram name="Sync.Attachments.UploadResponseCode"
     enum="CombinedHttpResponseAndNetErrorCode">
   <owner>maxbogue@chromium.org</owner>
@@ -56266,6 +56272,12 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="6" label="Reset retries"/>
 </enum>
 
+<enum name="SyncAttachmentStoreResult" type="int">
+  <int value="0" label="SUCCESS"/>
+  <int value="1" label="UNSPECIFIED_ERROR"/>
+  <int value="2" label="STORE_INITIALIZATION_FAILED"/>
+</enum>
+
 <enum name="SyncAuthError" type="int">
   <int value="0"
       label="Number of times clients have encountered an Auth error."/>