[SyncFS] Add a UMA to measure RTT in registration

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 df25d22f7d4b95c15a19b023300db6d10448dba9..11c7284bed76c1ff3b54e2db2db3b5cfee3bf5fb 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -27866,6 +27866,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="SyncFileSystem.RegisterOriginTime" units="milliseconds">
+  <owner>peria@chromium.org</owner>
+  <owner>tzik@chromium.org</owner>
+  <summary>
+    Round trip time to register the root path of tracking files in the Sync

[tzik, 2014/05/08 06:51:06]

Can this be like below for someone who isn't familiar to SyncFS?
"Time elapsed to register an app to SyncFilesystem.  Recorded for each
registration request by apps."

[peria, 2014/05/08 06:56:13]

Done.

+    FileSystem.
+  </summary>
+</histogram>
+
 <histogram name="SyncFileSystem.TrackerNumber">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>