[Android O] Add SystemNotification.ChannelBlocked histogram

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 456522f7be5e0967c3375a3b69d9d8f180430f4d..f1e35d7ac7c002a69e160014a3ed10a614fadd89 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -30263,6 +30263,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Mobile.SystemNotification.ChannelBlocked"
+    enum="SystemNotificationType">
+  <owner>awdf@chromium.org</owner>
+  <summary>
+    Android: Represents the number of system notifications that were blocked as
+    a result of their notification channel being blocked and could not be shown
+    by type. Only applies to devices on Android O and above.

[Peter Beverloo, 2017/04/20 17:30:29]

nit: s/and could not be shown//? It's redundant with the first part of the
sentence, where you explain that they were blocked.

[awdf, 2017/04/21 13:36:28]

changed it a bit to remove the redundancy, and make it clearer it's logged
whenever we try to show a notification but it's blocked by channel.

+  </summary>
+</histogram>
+
 <histogram name="Mobile.SystemNotification.Shown" enum="SystemNotificationType">
   <owner>dtrainor@chromium.org</owner>
   <summary>