Mark Net.AsyncRevalidation histograms as obsolete

I removed these histograms in March 2017 in
https://codereview.chromium.org/2764683002 but I forgot to mark them as
obsolete in histograms.xml.

Mark them as obsolete.

BUG=700568

Change-Id: I76e83d25ea45488ae8631c6402171dda3e61f987
Reviewed-on: https://chromium-review.googlesource.com/1161719Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580820}

tools/metrics/histograms/histograms.xml

@@ -49123,6 +49123,9 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Net.AsyncRevalidation.ReadError" enum="NetErrorCodes">
+  <obsolete>
+    Removed in March 2017. See https://crbug.com/700568.
+  </obsolete>
   <owner>ricea@chromium.org</owner>
   <summary>
     Counts of error codes received while reading the body of an async
@@ -49132,6 +49135,9 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Net.AsyncRevalidation.ResponseError" enum="NetErrorCodes">
+  <obsolete>
+    Removed in March 2017. See https://crbug.com/700568.
+  </obsolete>
   <owner>ricea@chromium.org</owner>
   <summary>
     Counts of error codes received while performing an async revalidation before
@@ -49141,6 +49147,9 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Net.AsyncRevalidation.Result" enum="AsyncRevalidationResult">
+  <obsolete>
+    Removed in March 2017. See https://crbug.com/700568.
+  </obsolete>
   <owner>ricea@chromium.org</owner>
   <summary>
     The result of an async revalidation resulting from application of the