[Android] Adding UMA for events in Background Task Scheduler

tools/metrics/histograms/enums.xml

Index: tools/metrics/histograms/enums.xml
diff --git a/tools/metrics/histograms/enums.xml b/tools/metrics/histograms/enums.xml
index d79e26a786af2e484ae09e48987d27754a5f7420..1e2b71ead10ef1d34652d5e1fbe65ebda8147ce2 100644
--- a/tools/metrics/histograms/enums.xml
+++ b/tools/metrics/histograms/enums.xml
@@ -2046,6 +2046,17 @@ uploading your change for review.  These are checked by presubmit scripts.
   <int value="5" label="Permission denied"/>
 </enum>
 
+<enum name="BackgroundTaskId" type="int">
+  <int value="0" label="Test task (should not appear)"/>
+  <int value="1" label="Omaha"/>
+  <int value="2" label="GCM Background Task"/>
+  <int value="3" label="Notification service task"/>
+  <int value="4" label="Webview minidump uploading task"/>
+  <int value="5" label="Chrome minidump uploading task"/>
+  <int value="6" label="Offlining pages task"/>
+  <int value="7" label="Offline page prefetch task"/>
+</enum>
+
 <enum name="BackgroundTracingState" type="int">
   <int value="0" label="Scenario activation requested"/>
   <int value="1" label="Scenario successfully activated"/>