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..3d964d4485a01fb9d74b73ee72181f9d87369e8d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -49554,6 +49554,22 @@ 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 resource. The

[Mark P, 2016/06/07 17:44:09]

(1)
I'm having trouble parsing this (first) sentence.  Should the word "started" be
removed?
(2)
I don't understand the second sentence in combination with the first.  The first
implies that the terminating event is "send a FetchEvent request".  The second
implies the terminating event is "finds a running worker or successfully starts
a worker."  These sound like different things to me.
(3)
receives is spelled as such
(4)
The third sentence "not recorded ... if the worker was not activated yet" is
confusing given the first two sentences.  I think this limitation should be
expressed more clearly in the first/second sentence, perhaps by saying something
like "find an active worker" up there, and in a following sentence saying that
If no worker is active, then this metric is not recorded.  (The registration and
sending the request will still take place.  The timing will not be emitted to
this histogram however.)
(5)
You can still have the devtools and New Tab Page restrictions at the end of the
description.  It would be nice to briefly justify the New Tab Page restriction,
just as you do the devtools restriction.

[horo, 2016/06/08 04:13:23]

Done.
Revised the summary.

+    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 in the following situations: 1) The worker was not
+    activated yet. This is intended to avoid including the time for the
+    activation. 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.
+  </summary>
+</histogram>
+
 <histogram name="ServiceWorker.ActivateEvent.Time" units="ms">
   <owner>shimazu@chromium.org</owner>
   <summary>
@@ -97058,6 +97074,22 @@ 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."/>

[Mark P, 2016/06/07 17:44:09]

I find these labels confusing, mainly because I thought the histogram was only
emitted for workers that were already activated.  Yet, by implication, only
three of these labels have already-active workers; two do not.  Why are those
emitted to the histogram?

[horo, 2016/06/08 04:13:23]

An "ACTIVE" Service Worker is different from a started Service Worker.
ACTIVE is a technical term in Service Worker spec.
There could be an ACTIVE stopped worker, and also a running worker which is not
activated.

+  <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