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 f8bf91ff5578043b45991f44a37d0caae7858097..23ac4c4ac7aa6792ddec6f15f4fc27b3bf9956f0 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -29420,6 +29420,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="ServiceWorker.ActivateEventExecutionTime" units="millisecond">
+  <owner>shimazu@chromium.org</owner>
+  <summary>Execution time of ServiceWorkerGlobalScope.onactivate.</summary>

[Mark P, 2014/09/29 18:00:36]

Does "Done" mean "Yes"?

[shimazu, 2014/09/30 01:31:09]

Oops, sorry. Yes.

+</histogram>
+
 <histogram name="ServiceWorker.Database.OpenResult"
     enum="ServiceWorkerDatabaseStatus">
   <owner>nhiroki@chromium.org</owner>
@@ -29465,6 +29470,16 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="ServiceWorker.FetchEventExecutionTime" units="millisecond">
+  <owner>shimazu@chromium.org</owner>
+  <summary>Execution time of ServiceWorkerGlobalScope.onfetch.</summary>
+</histogram>
+
+<histogram name="ServiceWorker.InstallEventExecutionTime" units="millisecond">
+  <owner>shimazu@chromium.org</owner>
+  <summary>Execution time of ServiceWorkerGlobalScope.oninstall.</summary>
+</histogram>
+
 <histogram name="ServiceWorker.ScriptSize" units="bytes">
   <owner>ksakamoto@chromium.org</owner>
   <summary>