Annotate CrashReport histograms as obsolete

After http://crrev/c/832942, these histograms are no longer being emitted

Bug: 795802
Change-Id: I7dfdf2ab640e23795670ab6226bd1124ddff0b38
Reviewed-on: https://chromium-review.googlesource.com/c/1315672Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605312}

tools/metrics/histograms/histograms.xml

@@ -16787,6 +16787,10 @@ uploading your change for review.
 </histogram>
 
 <histogram name="CrashReport.CrashBackgroundUploadDelay" units="ms">
+  <obsolete>
+    Deprecated as of 03/2018 when Chrome for iOS stopped uploading crash dumps
+    in the background.
+  </obsolete>
   <owner>olivierrobin@chromium.org</owner>
   <owner>pkl@chromium.org</owner>
   <summary>
@@ -16796,6 +16800,10 @@ uploading your change for review.
 </histogram>
 
 <histogram name="CrashReport.PendingReportsOnBackgroundWakeUp">
+  <obsolete>
+    Deprecated as of 03/2018 when Chrome for iOS stopped uploading crash dumps
+    in the background.
+  </obsolete>
   <owner>olivierrobin@chromium.org</owner>
   <owner>pkl@chromium.org</owner>
   <summary>