Added UMA stats to track the budget for any service worker which receives a

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 277aae703bb89c4c4397c61b8a7c5b0f3bd2f556..d8e8074956a084d1fbc8179147bd47c93b5f3e90 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -42371,6 +42371,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PushMessaging.BackgroundBudget">
+  <owner>harkness@chromium.org</owner>
+  <summary>
+    Whenever a Service Worker receives a push message, this records the budget
+    available to the service worker.
+  </summary>
+</histogram>
+
 <histogram name="PushMessaging.DeliveryStatus" enum="PushDeliveryStatus">
   <owner>johnme@google.com</owner>
   <owner>mvanouwerkerk@google.com</owner>
@@ -42401,6 +42409,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PushMessaging.SESForLowBudgetOrigin">
+  <owner>harkness@chromium.org</owner>
+  <summary>
+    When a Service Worker hits low budget when servicing a push message, this
+    records what the Site Engagement Service score is at that time.
+  </summary>
+</histogram>
+
+<histogram name="PushMessaging.SESForNoBudgetOrigin">
+  <owner>harkness@chromium.org</owner>
+  <summary>
+    When a Service Worker hits zero budget when servicing a push message, this
+    records what the Site Engagement Service score is at that time.
+  </summary>
+</histogram>
+
 <histogram name="PushMessaging.UnregistrationStatus"
     enum="PushUnregistrationStatus">
   <owner>johnme@google.com</owner>