Add support for tracking loading behavior of child frames.

chrome/browser/page_load_metrics/observers/css_scanning_page_load_metrics_observer.h

Index: chrome/browser/page_load_metrics/observers/css_scanning_page_load_metrics_observer.h
diff --git a/chrome/browser/page_load_metrics/observers/css_scanning_page_load_metrics_observer.h b/chrome/browser/page_load_metrics/observers/css_scanning_page_load_metrics_observer.h
index 7af74a0206f081c537809f6fb5df01b87585dc75..93f652a3b6bbbb4c7f183b4e048246e84fba0962 100644
--- a/chrome/browser/page_load_metrics/observers/css_scanning_page_load_metrics_observer.h
+++ b/chrome/browser/page_load_metrics/observers/css_scanning_page_load_metrics_observer.h
@@ -15,6 +15,14 @@ class CssScanningMetricsObserver
   ~CssScanningMetricsObserver() override;
 
   // page_load_metrics::PageLoadMetricsObserver:
+  ObservePolicy OnStart(content::NavigationHandle* navigation_handle,
+                        const GURL& currently_committed_url,
+                        bool started_in_foreground) override;
+  ObservePolicy OnHidden(
+      const page_load_metrics::PageLoadTiming& timing,
+      const page_load_metrics::PageLoadExtraInfo& info) override;
+  void OnLoadingBehaviorObserved(
+      const page_load_metrics::PageLoadExtraInfo& info) override;
   void OnFirstContentfulPaint(
       const page_load_metrics::PageLoadTiming& timing,
       const page_load_metrics::PageLoadExtraInfo& info) override;
@@ -23,6 +31,8 @@ class CssScanningMetricsObserver
       const page_load_metrics::PageLoadExtraInfo& info) override;
 
  private:
+  bool css_preload_found_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(CssScanningMetricsObserver);
 };