ServiceWorker: Add UMA to measure execution times of oninstall/onfetch

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 8b485444fbacbb3227cfd430de9f8db265ed9d9c..92952ae069fa599a65a07261ce31bf6d4fd0bf21 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -29341,6 +29341,23 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="ServiceWorker.FetchEventExecutionTime" units="milliseconds">
+  <owner>shimazu@chromium.org</owner>
+  <summary>
+    Execution time of onfetch event. This is mainly composed by a execution of

[Mark P, 2014/09/26 16:20:48]

For my information, are these recorded regardless of how the event terminates
(e.g., success or throws error)?  If so, no need to make any revisions.

[shimazu, 2014/09/29 08:15:47]

Done.

+    onfetch function in ServiceWorker script.
+  </summary>
+</histogram>
+
+<histogram name="ServiceWorker.RegistrationTime" units="milliseconds">
+  <owner>shimazu@chromium.org</owner>
+  <summary>
+    Execution time of registration. This is composed by caching SW script and
+    oninstall event. If much data is cached or much calculation is in oninstall,
+    this will increase.
+  </summary>
+</histogram>
+
 <histogram name="ServiceWorker.ScriptSize" units="bytes">
   <owner>ksakamoto@chromium.org</owner>
   <summary>