[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 f37ddeead3e26558fb0e9a41fbde0b68210d0978..83256659cf1e8f5a13c4150ddd333247aaa1c355 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -30155,6 +30155,16 @@ 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 by type that we
+    attempted to show but were blocked, due to their notification channel being
+    blocked. Only applies to devices on Android O and above.
+  </summary>
+</histogram>
+
 <histogram name="Mobile.SystemNotification.Shown" enum="SystemNotificationType">
   <owner>dtrainor@chromium.org</owner>
   <summary>
@@ -31487,8 +31497,8 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="Navigation.MainFrameSchemeOTR" enum="NavigationScheme">
   <owner>elawrence@chromium.org</owner>
   <owner>estark@chromium.org</owner>
-  <summary>The scheme of the URL for each main-frame navigation while in
-    incognito.
+  <summary>
+    The scheme of the URL for each main-frame navigation while in incognito.
   </summary>
 </histogram>