Prerender: Add FCP histograms to nostate browsertests.

chrome/browser/prerender/prerender_manager.cc

Index: chrome/browser/prerender/prerender_manager.cc
diff --git a/chrome/browser/prerender/prerender_manager.cc b/chrome/browser/prerender/prerender_manager.cc
index 090a5539d984ca5aeeb50ab3269dac2aad32dcc5..aa8933f4096e2d5398092f1db90e0915e6a61e47 100644
--- a/chrome/browser/prerender/prerender_manager.cc
+++ b/chrome/browser/prerender/prerender_manager.cc
@@ -615,15 +615,15 @@ void PrerenderManager::RecordNoStateFirstContentfulPaint(const GURL& url,
       std::remove_if(prefetches_.begin(), prefetches_.end(),
                      [url](const NavigationRecord& r) { return r.url == url; }),
       prefetches_.end());
-}
 
-void PrerenderManager::RecordPerceivedFirstContentfulPaint(
-    content::WebContents* web_contents,
-    base::TimeDelta first_contentful_paint) {
   for (auto& observer : observers_) {
     observer->OnFirstContentfulPaint();

[pasko, 2016/11/18 15:56:56]

can we notify observers just after recording histograms? The manipulations of
prefetches_ above might be potentially non-instant, so in case observers ever
get sensitive to precision, we would do our best here. I think this does not
matter in practice, but looks safer and raises less questions.

[mattcary, 2016/11/18 16:13:27]

SGTM. I moved it to the end of the function for a similar reasons, but your
suggestion is more precise.

Note that it now occurs to me that what we are really using these observers for
is to wait for the FCP histograms, not actually the FCP. That only affects
tests, though, and it seems like it might be confusing to emphasize that detail.

   }
+}
 
+void PrerenderManager::RecordPerceivedFirstContentfulPaint(
+    content::WebContents* web_contents,
+    base::TimeDelta first_contentful_paint) {
   PrerenderTabHelper* tab_helper =
       PrerenderTabHelper::FromWebContents(web_contents);
   DCHECK(tab_helper);
@@ -638,6 +638,10 @@ void PrerenderManager::RecordPerceivedFirstContentfulPaint(
     histograms_->RecordPerceivedFirstContentfulPaintStatus(tab_helper->origin(),
                                                            false);
   }
+
+  for (auto& observer : observers_) {

[pasko, 2016/11/18 15:56:56]

this moved too, does anything depend on firing it later?

[mattcary, 2016/11/18 16:13:27]

I moved this to be consistent with the other FCP (ie, at the end). At any rate,
it happily matches your other suggestion.

+    observer->OnFirstContentfulPaint();
+  }
 }
 
 // static