Add NavigationToFirstContentfulPaint UMA for ServiceWorker controlled pages.

chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.h"
 
 #include <stddef.h>
 #include <stdint.h>
 #include <utility>
 
@@ skipping 79 matching lines @@
     "PageLoad.Timing2.ParseBlockedOnScriptLoad.Background";
 const char kBackgroundHistogramParseBlockedOnScriptLoadParseComplete[] =
     "PageLoad.Timing2.ParseBlockedOnScriptLoad.ParseComplete.Background";
 const char kBackgroundHistogramParseBlockedOnScriptLoadDocumentWrite[] =
     "PageLoad.Timing2.ParseBlockedOnScriptLoadFromDocumentWrite.Background";
 const char
     kBackgroundHistogramParseBlockedOnScriptLoadDocumentWriteParseComplete[] =
         "PageLoad.Timing2.ParseBlockedOnScriptLoadFromDocumentWrite."
         "ParseComplete.Background";
 
+const char kServiceWorkerHistogramFirstContentfulPaint[] =

[Bryan McQuade, 2016/04/12 20:29:10]

I wonder if this should instead be under PageLoad.Clients.ServiceWorker.*

Take a look at
https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/pag...

If we only anticipate ever adding a single service worker specific metric, then
maybe this is ok. But my hunch is we may want to measure various SW metrics, in
which case having a separate observer and client makes more sense.

+    "PageLoad.Timing2.NavigationToFirstContentfulPaint.ServiceWorker";
+
 const char kHistogramFirstContentfulPaintHigh[] =
     "PageLoad.Timing2.NavigationToFirstContentfulPaint.HighResolutionClock";
 const char kHistogramFirstContentfulPaintLow[] =
     "PageLoad.Timing2.NavigationToFirstContentfulPaint.LowResolutionClock";
 
 const char kHistogramFirstBackground[] =
     "PageLoad.Timing2.NavigationToFirstBackground";
 const char kHistogramFirstForeground[] =
     "PageLoad.Timing2.NavigationToFirstForeground";
 
@@ skipping 165 matching lines @@
                           timing.first_contentful_paint);
       // Bucket these histograms into high/low resolution clock systems. This
       // might point us to directions that will de-noise some UMA.
       if (base::TimeTicks::IsHighResolution()) {
         PAGE_LOAD_HISTOGRAM(internal::kHistogramFirstContentfulPaintHigh,
                             timing.first_contentful_paint);
       } else {
         PAGE_LOAD_HISTOGRAM(internal::kHistogramFirstContentfulPaintLow,
                             timing.first_contentful_paint);
       }
+      if (!timing.worker_start.is_null()) {

[Bryan McQuade, 2016/04/12 20:29:10]

I'm disinclined to use absolute times in our metrics.

It sounds like your goal is to communicate whether or not a worker was used for
a page. For that, you may want to use the new WebLoadingBehaviorFlag to annotate
page load metrics with whether a SW was active for that page on the blink side.
You can see how that works here: https://codereview.chromium.org/1846143003 and
in this pending change for the metrics side:
https://codereview.chromium.org/1857443002

You may want to email Kinuko and Charles to see which they think makes sense. My
hunch is that's better than sending a base::Time which is really only intended
to be used as a flag to indicate that a worker was used for a page load.

+        PAGE_LOAD_HISTOGRAM(
+            internal::kServiceWorkerHistogramFirstContentfulPaint,
+            timing.first_contentful_paint);
+      }
       PAGE_LOAD_HISTOGRAM(internal::kHistogramDomLoadingToFirstContentfulPaint,
                           timing.first_contentful_paint - timing.dom_loading);
     } else {
       PAGE_LOAD_HISTOGRAM(internal::kBackgroundHistogramFirstContentfulPaint,
                           timing.first_contentful_paint);
     }
   }
   if (!timing.parse_start.is_zero()) {
     const bool incomplete_parse_in_foreground =
         timing.parse_stop.is_zero() && info.started_in_foreground &&
@@ skipping 82 matching lines @@
       RapporHistogramBucketIndex(timing.first_contentful_paint);
   sample->SetFlagsField("Bucket", uint64_t(1) << bucket_index,
                         kNumRapporHistogramBuckets);
   // The IsSlow flag is just a one bit boolean if the first contentful paint
   // was > 10s.
   sample->SetFlagsField("IsSlow",
                         timing.first_contentful_paint.InSecondsF() >= 10, 1);
   rappor_service->RecordSampleObj(internal::kRapporMetricsNameCoarseTiming,
                                   std::move(sample));
 }