Add option to disallow the discarding of a tab that was previously discarded.

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 e9b82d75dbb11aafe93d7af740f87cd277ce45d4..4e21f7f1983490984d926d1db6977ae45487314f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -46986,6 +46986,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Tab.Discarding.DiscardCount" units="Discards">
+  <owner>georgesak@chromium.org</owner>
+  <summary>
+    Metric to track the number of discards tabs have gone through. Each time a
+    tab is discarded, this histogram is recorded. Therefore, this metric is
+    cumulative, ie. as a tab gets discarded over and over, it gets a hit in each
+    bin (1, 2, 3...).

[rkaplow, 2015/09/11 21:07:03]

this is a strange way of recording it. Why not just increment every time there's
a discard? With this the sum / mean / etc doesn't make sense.

[Georges Khalil, 2015/09/14 18:17:48]

Discussed offline, keeping as is.

+  </summary>
+</histogram>
+
 <histogram name="Tab.EvictedTabWasActive" enum="Boolean">
   <owner>lliabraa@chromium.org</owner>
   <summary>