Introduce ServiceWorker.ActivatedWorkerPreparationForMainFrame.Time UMA.

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 eeea9193acb9d4e9a4283bcdc7b6759c2d6d35be..73f89c6fcb497fe4724dfa4be41b6bee8ff62d47 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -49521,6 +49521,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ServiceWorker.ActivatedWorkerPreparationForMainFrame.Time"
+    units="ms">
+  <owner>horo@chromium.org</owner>
+  <summary>
+    The time taken to prepare an activated Service Worker for a main frame

[falken, 2016/06/07 03:46:12]

I think "prepare" is ambiguous. It could sound like the time taken to find a
registration and wait for a waiting worker to become activated. Could this
explanation:
- Explain exactly when the measurement starts and ends?
- Explain that it's not recorded unless the worker was already activated?

[horo, 2016/06/07 05:58:12]

Done.

+    fetch. The metric is not recorded if the Service Worker was started for the
+    fetch and DevTools was ever attached to the Service Worker during startup.

[falken, 2016/06/07 03:46:12]

"not recorded if the SW was started for the fetch" should be "not recorded if
the SW was already started" ?

[horo, 2016/06/07 05:58:12]

No.
When the SW was already started, it is recorded as
ServiceWorker.ActivatedWorkerPreparationForMainFrame.Time_RunningWorker.

+    NTP is excluded from this metric.
+  </summary>
+</histogram>
+
 <histogram name="ServiceWorker.ActivateEvent.Time" units="ms">
   <owner>shimazu@chromium.org</owner>
   <summary>
@@ -96982,6 +96993,17 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="ServiceWorker.StartWorker.Time"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="ServiceWorker.WorkerPreparationMode">
+  <suffix name="StartWorkerDuringStartup" label="StartWorkerDuringStartup"/>
+  <suffix name="StartWorkerNewProcess" label="StartWorkerNewProcess"/>
+  <suffix name="StartWorkerExistingProcess" label="StartWorkerExistingProcess"/>
+  <suffix name="SartingWorker" label="SartingWorker"/>

[falken, 2016/06/07 03:46:12]

s/Sarting/Starting/g

[horo, 2016/06/07 05:58:12]

Done.

+  <suffix name="RunningWorker" label="RunningWorker"/>
+  <suffix name="StoppingWorker" label="StoppingWorker"/>
+  <affected-histogram
+      name="ServiceWorker.ActivatedWorkerPreparationForMainFrame.Time"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="ServiceWorkerCache.Cache" separator=".">
   <suffix name="Batch"
       label="The time to perform the 'batch' operation on a ServiceWorker