Add NavigationToFirstContentfulPaint UMA for ServiceWorker controlled pages.

components/page_load_metrics/renderer/metrics_render_frame_observer.cc

Index: components/page_load_metrics/renderer/metrics_render_frame_observer.cc
diff --git a/components/page_load_metrics/renderer/metrics_render_frame_observer.cc b/components/page_load_metrics/renderer/metrics_render_frame_observer.cc
index 260d3b6285d48c68a63ce22b127e9b70890555ad..812c281152bc4132907a2328da8162658194096c 100644
--- a/components/page_load_metrics/renderer/metrics_render_frame_observer.cc
+++ b/components/page_load_metrics/renderer/metrics_render_frame_observer.cc
@@ -41,6 +41,8 @@ void MetricsRenderFrameObserver::DidChangePerformanceTiming() {
 
 void MetricsRenderFrameObserver::DidObserveLoadingBehavior(
     blink::WebLoadingBehaviorFlag behavior) {
+  if (!ShouldSendMetrics())

[Charlie Harrison, 2016/04/14 13:29:56]

Actually, I think I retract my comment. This won't work for an in-page
navigation.

Additionally, the logic in DidChangePerformanceTiming looks at
!page_timing_metrics_sender_, so perhaps it's best to do the conservative check
for consistency. I'm adding this check because clusterfuzz found the bug in my
CL.

[horo, 2016/04/14 14:01:30]

Changed to check page_timing_metrics_sender_

+    return;
   DCHECK(page_timing_metrics_sender_);
   page_timing_metrics_sender_->DidObserveLoadingBehavior(behavior);
 }