Add NavigationToFirstContentfulPaint UMA for ServiceWorker controlled pages.

components/page_load_metrics/renderer/metrics_render_frame_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 "components/page_load_metrics/renderer/metrics_render_frame_observer.h"
 
 #include <string>
 
 #include "base/time/time.h"
 #include "base/timer/timer.h"
@@ skipping 23 matching lines @@
     : content::RenderFrameObserver(render_frame) {}
 
 MetricsRenderFrameObserver::~MetricsRenderFrameObserver() {}
 
 void MetricsRenderFrameObserver::DidChangePerformanceTiming() {
   SendMetrics();
 }
 
 void MetricsRenderFrameObserver::DidObserveLoadingBehavior(
     blink::WebLoadingBehaviorFlag behavior) {
-  DCHECK(page_timing_metrics_sender_);
-  page_timing_metrics_sender_->DidObserveLoadingBehavior(behavior);
+  if (page_timing_metrics_sender_)

[Charlie Harrison, 2016/04/13 12:59:13]

This worries me slightly. We try to scope page_timing_metrics_sender_ to a
committed page, so that we never send IPC for a provisional load.

If you are sometimes calling DidObserveLoadingBehavior before the commit point
(from contents point of view), then won't you miss this window occasionally and
not update the behavior?

Looking at the code now, it seems like you're instrumenting after content should
know we've commit. Still, can you add a comment explaining when the sender_ will
be invalid?

[horo, 2016/04/14 02:19:13]

DidCommitProvisionalLoad() doesn't create page_timing_metrics_sender_ if
ShouldSendMetrics() returns false.
For example, when the mime type isn't HTML mime type,
page_timing_metrics_sender_ is null when DidObserveLoadingBehavior() is called.

So I think we should check ShouldSendMetrics() or page_timing_metrics_sender_
here.
But as you say, checking only page_timing_metrics_sender_ may cause IPCs for a
provisional load.
So I changed to check ShouldSendMetrics() here.

+    page_timing_metrics_sender_->DidObserveLoadingBehavior(behavior);
 }
 
 void MetricsRenderFrameObserver::DidCommitProvisionalLoad(
     bool is_new_navigation,
     bool is_same_page_navigation) {
   // Same-page navigations (e.g. an in-document navigation from a fragment
   // link) aren't full page loads, since they don't go to network to load the
   // main HTML resource. DidStartProvisionalLoad doesn't get invoked for same
   // page navigations, so we may still have an active
   // page_timing_metrics_sender_ at this point.
@@ skipping 84 matching lines @@
   return make_scoped_ptr(new base::OneShotTimer);
 }
 
 bool MetricsRenderFrameObserver::HasNoRenderFrame() const {
   bool no_frame = !render_frame() || !render_frame()->GetWebFrame();
   DCHECK(!no_frame);
   return no_frame;
 }
 
 }  // namespace page_load_metrics