[page_load_metrics] Get navigation_start from NavigationHandle

content/public/browser/navigation_handle.h

Index: content/public/browser/navigation_handle.h
diff --git a/content/public/browser/navigation_handle.h b/content/public/browser/navigation_handle.h
index 3b2bbadda772a2f31a97b5faff489b0b730728c9..cc96381fbaa227d5d80dafa1d0c56e0adcd336c6 100644
--- a/content/public/browser/navigation_handle.h
+++ b/content/public/browser/navigation_handle.h
@@ -39,6 +39,10 @@ class CONTENT_EXPORT NavigationHandle {
   // The WebContents the navigation is taking place in.
   WebContents* GetWebContents();
 
+  // The time the navigation started, recorded either in the renderer or in the
+  // browser process. Corresponds to Navigation Timing API.
+  virtual const base::TimeTicks& NavigationStart() = 0;
+
   // Parameters available at network request start time ------------------------
   //
   // The following parameters are only available when the network request is