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 28d90ca0cba04971f8185933e98c46b9baf60fd6..087d4ccd34b47455a828f445a50c350f13d09e32 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -49554,6 +49554,21 @@ 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 find a Service Worker registration and wait for a worker
+    started to send a FetchEvent request for a main frame HTML file. The
+    measurement starts when the browser process recieves the request and ends
+    when it finds a running worker or it successfully starts a worker. The
+    metric is not recorded unless the worker was already activated to avoid
+    including the time for the activation. The metric is not recorded if the
+    worker was started for the fetch and DevTools was ever attached to the

[falken, 2016/06/07 06:20:30]

Sorry I don't understand what "is not recorded if the worker was started for the
fetch" means, given
ServiceWorker.ActivatedWorkerPreparationForMainFrame.Time_RunningWorker is
intended to be recorded.

[horo, 2016/06/07 06:58:48]

Updated the summary.

+    worker during the startup. NTP is excluded from this metric.
+  </summary>
+</histogram>
+
 <histogram name="ServiceWorker.ActivateEvent.Time" units="ms">
   <owner>shimazu@chromium.org</owner>
   <summary>
@@ -97058,6 +97073,17 @@ 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="StartingWorker" label="StartingWorker"/>
+  <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