Mark some histograms as obsolete.

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 01e6809e03ca3e89f81aeb43d388576ee5f0ddf2..19d4c6ee9388c66653af3f73c4ebbeb4622fbc1c 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -35520,6 +35520,9 @@ Therefore, the affected-histogram name has to have at least one dot in it.
 </histogram>
 
 <histogram name="UMA.FieldTrialsEnabledBenchmarking" enum="BooleanUsage">
+  <obsolete>
+    Deprecated 2012. No longer tracked.
+  </obsolete>
   <owner>asvitkine@chromium.org</owner>
   <summary>
     Log whether the --enable-benchmarking flag was set, which causes field
@@ -35720,6 +35723,9 @@ Therefore, the affected-histogram name has to have at least one dot in it.
 </histogram>
 
 <histogram name="UMA.UploadResponseStatus.XML" enum="UmaUploadResponseStatus">
+  <obsolete>
+    Deprecated 2013. No longer tracked.
+  </obsolete>
   <owner>asvitkine@chromium.org</owner>
   <summary>
     For each upload to the XML (v1) UMA server, log whether the upload was
@@ -35736,6 +35742,9 @@ Therefore, the affected-histogram name has to have at least one dot in it.
 </histogram>
 
 <histogram name="UMA.XMLNodeDumpTime" units="milliseconds">
+  <obsolete>
+    Deprecated 2013. No longer tracked.
+  </obsolete>
   <owner>asvitkine@chromium.org</owner>
   <summary>
     The time spent in converting the XML tree into a character buffer when
@@ -35744,6 +35753,9 @@ Therefore, the affected-histogram name has to have at least one dot in it.
 </histogram>
 
 <histogram name="UMA.XMLWriterDestructionTime" units="milliseconds">
+  <obsolete>
+    Deprecated 2013. No longer tracked.
+  </obsolete>
   <owner>asvitkine@chromium.org</owner>
   <summary>
     The time spent in freeing the XML writer and tree when closing a metrics log