Add RAPPOR metrics of ServiceWorker.ControlledPage

tools/metrics/rappor/rappor.xml

Index: tools/metrics/rappor/rappor.xml
diff --git a/tools/metrics/rappor/rappor.xml b/tools/metrics/rappor/rappor.xml
index 62ae0d58e987d7cf3b25d719a31cd8c926159195..c98501f8e40cb60291e0bf676f204bd9ee3cd30d 100644
--- a/tools/metrics/rappor/rappor.xml
+++ b/tools/metrics/rappor/rappor.xml
@@ -145,6 +145,14 @@ components/rappor/rappor_service.cc.
   </summary>
 </rappor-metric>
 
+<rappor-metric name="ServiceWorker.ControlledPageLoad" type="ETLD_PLUS_ONE">
+  <owner>horo@chromium.org</owner>
+  <summary>
+    The domain and registry of the top level URL of a page which is controlled

[falken, 2015/02/25 01:43:59]

I'm not sure top level URL makes sense here, if SW controls an iframe, we'll
record the iframe's url. What's top level mean anyhow?

[horo, 2015/02/25 02:27:00]

I thought that "the top level URL" means the origin of URL.
But it was my misunderstanding.
I removed "top level".

+    by a ServiceWorker. Recorded when a page is loaded.
+  </summary>
+</rappor-metric>
+
 <rappor-metric name="Settings.FirstStartupPage" type="ETLD_PLUS_ONE">
   <owner>holte@chromium.org</owner>
   <summary>