Record some foreign fetch registration metrics.

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 4bd5e86b0386cb9748d21c8fc5fa7ba483fadd51..30c5baa6de8efcfcdcbda571e96e57cc115b98e0 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -51976,6 +51976,25 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ServiceWorker.ForeignFetch.OriginCount">
+  <owner>mek@chromium.org</owner>
+  <summary>
+    The number of origins a particular service worker with foreign fetch
+    registrations the worker will intercept fetches from. A value of zero
+    indicates that the foreign fetch service worker intercepts requests from any
+    origin. Recorded t the end of a successful install event, if the service

[Mark P, 2016/08/11 22:42:18]

"t" -> "at"?

[Marijn Kruisselbrink, 2016/08/12 20:33:23]

fixed

+    worker register for foreign fetch.

[Mark P, 2016/08/11 22:42:18]

Can a service worker register for foreign fetch but not claim any origins?

[Marijn Kruisselbrink, 2016/08/12 20:33:23]

No, hence why "all origins" is internally represented as an empty list of
origins.

+  </summary>
+</histogram>
+
+<histogram name="ServiceWorker.ForeignFetch.SubScopeCount">
+  <owner>mek@chromium.org</owner>
+  <summary>
+    The number of subscopes a particular service worker registered for. Recorded

[Mark P, 2016/08/11 22:42:18]

Please briefly explain what a subscope is.

[Marijn Kruisselbrink, 2016/08/12 20:33:23]

changed it to "foreign fetch scopes". And I'm not sure it makes sense to really
explain every concept that is referenced in a histogram in great detail. It
doesn't seem feasible to try to make these descriptions understandable for
people that don't know anything about the feature they're logging stuff about.

+    at the end of every succesful install event.

[Mark P, 2016/08/11 22:42:18]

nit: every -> a
(to be identical to the sentence you used in the other histogram)

[Marijn Kruisselbrink, 2016/08/12 20:33:23]

I think this is clearer by not having this be identical to the other histogram.
After all the other histograms is only recorded for some install events (namely
those where foreign fetch registration took place), but this histogram is
recorded for every install event.

[Mark P, 2016/08/16 22:27:14]

Ah, thank you for pointing out the difference.

+  </summary>
+</histogram>
+
 <histogram name="ServiceWorker.ForeignFetchEvent.Fallback.Time" units="ms">
   <owner>mek@chromium.org</owner>
   <summary>