Added UMA for usage of BudgetAPI calls.

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 9bc93150da2a4b1e2722d224bde95790dda6ec31..dbdb356788f0cffaa2880514b59506c3daa30fa7 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -5085,6 +5085,28 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>A bubble was given to the bubble manager but not shown.</summary>
 </histogram>
 
+<histogram name="BudgetAPI.QueryBudget" units="budget">
+  <owner>harkness@chromium.org</owner>
+  <summary>
+    Whenever the Budget API framework receives a query for the current budget,
+    this records the total budget available to the origin, which is an internal
+    Chrome value for the amount of background processing an origin can do
+    without visibly alerting the user. Scale for the budget is 0 to 100. Care
+    should be taken when drawing conclusions from this metric, as budget is
+    based on both engagement and usage, so could vary significantly based on
+    user behaviour and origin.
+  </summary>
+</histogram>
+
+<histogram name="BudgetAPI.Reserve" enum="BooleanSuccess">

[Alexei Svitkine (slow), 2016/11/29 16:45:35]

This seems to be adding a new top-level prefix in histogram names.

This is generally discouraged unless you expect this will be used a lot for new
metrics. Can this be nested under an existing prefix?

[harkness, 2016/11/30 11:31:52]

Currently, the API is only provided for silent push, however we intend to add
support for other background processing in the future. 

I would imagine that we would eventually add another 3-4 metrics under this
heading.  we could, conceivably, move the metric under PushMessaging, but that
only makes sense for the silent push case, and would be very odd when we add
other  functionality. 

[Alexei Svitkine (slow), 2016/12/01 18:25:10]

Since this is a web platform API, how about Blink.BudgetAPI.*?

[harkness, 2016/12/02 13:12:38]

Good suggestion, I've updated it to Blink.BudgetAPI.*

+  <owner>harkness@chromium.org</owner>
+  <summary>
+    Whenever the Budget API framework receives a request to reserve budget for a
+    future background operation, this records whether the reserve request
+    succeeded or not.
+  </summary>
+</histogram>
+
 <histogram name="Canvas.ContextType" enum="CanvasContextType">
   <owner>junov@chromium.org</owner>
   <owner>kbr@chromium.org</owner>