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 71d03b9b142847750ecf7df1571956faaddef0ec..ced23b3ba5c29945c274b7da9eca75b4f72a11ec 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -49554,6 +49554,30 @@ 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 for the browser to find and possibly start an active worker
+    to which to dispatch a FetchEvent for a main frame resource request. For

[Mark P, 2016/06/08 20:08:46]

delete "to which"

[horo, 2016/06/08 22:54:51]

Done.

+    context, a FetchEvent can only be dispatched to an ACTIVATED worker that is
+    running (it has been successfully started). The measurements starts when the
+    browser process receives the request. The browser then finds the worker
+    appropriate for this request (if there is none, this metric is not
+    recorded). If that worker is already started, the browser process can send
+    the request to it, so the measurement ends quickly. Otherwise the browser
+    process has to start the worker and the measurement ends when the worker is
+    successfully started. The metric is not recorded in the following

[Mark P, 2016/06/08 20:08:45]

after "started", perhaps add "(we do not include in the time it takes for the
worker to become ACTIVATED)" (if true)

[horo, 2016/06/08 22:54:51]

Done.

+    situations: 1) The worker was in state INSTALLED or ACTIVATING, and the
+    browser had to wait for it to become ACTIVATED. This is to avoid including
+    the time to execute the activate event handlers in the worker's script. 2)
+    The worker was started for the fetch AND DevTools was attached during
+    startup. This is intended to avoid including the time for debugging. 3) The
+    request is for New Tab Page. This is because it tends to dominate the stats
+    and makes the results largely skewed.
+  </summary>
+</histogram>
+
 <histogram name="ServiceWorker.ActivateEvent.Time" units="ms">
   <owner>shimazu@chromium.org</owner>
   <summary>
@@ -97069,6 +97093,22 @@ 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="Started a worker during browser startup."/>
+  <suffix name="StartWorkerNewProcess"
+      label="Started a worker in a new process."/>
+  <suffix name="StartWorkerExistingProcess"
+      label="Started a worker in an existing process."/>
+  <suffix name="StartingWorker"
+      label="The activated worker was already starting up."/>
+  <suffix name="RunningWorker"
+      label="The activated worker was already running."/>
+  <suffix name="StoppingWorker" label="The activated worker was stopping."/>
+  <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