Deprecate the LowMemoryMargin histogram suffix.

LowMemoryMargin is a per-platform constant. There is no need to have
a LowMemoryMargin suffix on histograms. This suffix should have been
deprecated in 2013 https://chromiumcodereview.appspot.com/13993008

Bug: 775644
Change-Id: I2a809be0dadf3351d7a2f37efe981ac304cd8cdf
Reviewed-on: https://chromium-review.googlesource.com/806357Reviewed-by: Gayane Petrosyan <gayane@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521366}

tools/metrics/histograms/histograms.xml

@@ -102648,6 +102648,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram_suffixes>
 
 <histogram_suffixes name="LowMemoryMargin" separator="_">
+  <obsolete>
+    Deprecated 12/2017. The low memory margin is a per-platform constant.
+  </obsolete>
   <suffix name="default" label="Low memory margin set to the system default"/>
   <suffix name="off" label="Low memory notification disabled"/>
   <suffix name="0mb" label="Low memory margin set to 0MB"/>