predictors: Mark before_first_contentful_paint for resources fetched before fcp.

chrome/browser/predictors/resource_prefetch_predictor.cc

Index: chrome/browser/predictors/resource_prefetch_predictor.cc
diff --git a/chrome/browser/predictors/resource_prefetch_predictor.cc b/chrome/browser/predictors/resource_prefetch_predictor.cc
index dd33e10874d8259c5ec52ab6f7995777dbac7b91..083b0697110520c6a30217bb5bb37ae5432a8ac9 100644
--- a/chrome/browser/predictors/resource_prefetch_predictor.cc
+++ b/chrome/browser/predictors/resource_prefetch_predictor.cc
@@ -578,6 +578,18 @@ void ResourcePrefetchPredictor::RecordMainFrameLoadComplete(
   }
 }
 
+void ResourcePrefetchPredictor::RecordFirstContentfulPaint(
+    const NavigationID navigation_id) {
+  NavigationMap::iterator nav_it =
+      inflight_navigations_.find(navigation_id);
+  if (nav_it == inflight_navigations_.end()) return;

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

nit:
Return statement should occupy a separate line.
Please, run `git cl format` to automatically reformat code according to style
guides.

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

Done.

+
+  for (auto& request_summary : nav_it->second->subresource_requests) {
+    request_summary.before_first_contentful_paint = true;

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

You can't assume that Observer gets an event at the same time when first
contentful paint happens. 
The flow is following:
(1) Navigation start =====> (2) Navigation commit =====> (3) First contentful
paint =====> (4) PageLoadMetricsObserver::OnFirstContentfulPaint() is called

I've made a little experiment showing that the time between (3) and (4) could be
significant (>1s)
You need to use PageLoadTiming::first_contentful_paint to determine when (3)
exactly happened.

And it could be very tricky if OnLoad() event handlers complete between (3) and
(4). We'll write everything in the database assuming that all resources are
before fcp while some resources could be loaded after (3).

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

Maybe fixed. How likely is it that OnFirstContentfulPaint could be called after
onload? If it's sufficiently rare, it should work fine to ignore the
possibility.

[alexilin, 2017/03/27 15:31:59]

Well, it's definitely not rare for "fast" pages for which the time between fcp
and onload is short. Then it could happen that onload appears before
OnFirstContentfulPaint event. Examples: google properties, facebook, wikipedia. 
We don't target them, though. So, maybe, it's fine and isn't worth complexity.

+  }
+}
+
+
 void ResourcePrefetchPredictor::StartPrefetching(const GURL& url,
                                                  PrefetchOrigin origin) {
   TRACE_EVENT1("browser", "ResourcePrefetchPredictor::StartPrefetching", "url",
@@ -1146,6 +1158,8 @@ void ResourcePrefetchPredictor::LearnNavigation(
         continue;
 
       ResourceData* resource_to_add = data.add_resources();
+      resource_to_add->set_before_first_contentful_paint(
+          summary.before_first_contentful_paint);
       resource_to_add->set_resource_url(summary.resource_url.spec());
       resource_to_add->set_resource_type(
           static_cast<ResourceData::ResourceType>(summary.resource_type));