predictors: Mark before_first_contentful_paint for resources fetched before fcp.

chrome/browser/page_load_metrics/observers/resource_prefetch_predictor_page_load_metrics_observer.cc

Index: chrome/browser/page_load_metrics/observers/resource_prefetch_predictor_page_load_metrics_observer.cc
diff --git a/chrome/browser/page_load_metrics/observers/resource_prefetch_predictor_page_load_metrics_observer.cc b/chrome/browser/page_load_metrics/observers/resource_prefetch_predictor_page_load_metrics_observer.cc
index 0d383adeb10375f5966d33cf62b475f3e58c0d5a..38b982d4ecaf86ce73b0b7e5934fbd29a1791733 100644
--- a/chrome/browser/page_load_metrics/observers/resource_prefetch_predictor_page_load_metrics_observer.cc
+++ b/chrome/browser/page_load_metrics/observers/resource_prefetch_predictor_page_load_metrics_observer.cc
@@ -30,14 +30,16 @@ ResourcePrefetchPredictorPageLoadMetricsObserver::CreateIfNeeded(
           web_contents->GetBrowserContext());
   if (!predictor)
     return nullptr;
+  predictors::NavigationID navigation_id(web_contents);

[alexilin, 2017/03/17 14:46:46]

I'm afraid it won't work correctly.
The observer is created before navigation commit, at this point we don't know
navigation final destination. NavigationID contains main_frame_url it has to be
created after navigation commit.

You could keep a pointer to web_contents inside the class and create
NavigationID directly in OnFirstContentfulPaint().

[trevordixon, 2017/03/27 12:30:08]

Done.

   return base::MakeUnique<ResourcePrefetchPredictorPageLoadMetricsObserver>(
-      predictor);
+      predictor, navigation_id);
 }
 
 ResourcePrefetchPredictorPageLoadMetricsObserver::
     ResourcePrefetchPredictorPageLoadMetricsObserver(
-        predictors::ResourcePrefetchPredictor* predictor)
-    : predictor_(predictor) {
+        predictors::ResourcePrefetchPredictor* predictor,
+        predictors::NavigationID navigation_id)
+    : predictor_(predictor), navigation_id_(navigation_id) {
   DCHECK(predictor_);
 }
 
@@ -65,6 +67,7 @@ ResourcePrefetchPredictorPageLoadMetricsObserver::OnHidden(
 void ResourcePrefetchPredictorPageLoadMetricsObserver::OnFirstContentfulPaint(
     const page_load_metrics::PageLoadTiming& timing,
     const page_load_metrics::PageLoadExtraInfo& extra_info) {
+  predictor_->RecordFirstContentfulPaint(navigation_id_);
   PAGE_LOAD_HISTOGRAM(
       internal::kHistogramResourcePrefetchPredictorFirstContentfulPaint,
       timing.first_contentful_paint.value());