Add Perceived PageLoad Time (PPLT) metrics for Prerender Experiments,...

chrome/browser/tab_contents/tab_contents.h

Index: chrome/browser/tab_contents/tab_contents.h
===================================================================
--- chrome/browser/tab_contents/tab_contents.h	(revision 72357)
+++ chrome/browser/tab_contents/tab_contents.h	(working copy)
@@ -1166,6 +1166,10 @@
   // System time at which the current load was started.
   base::TimeTicks current_load_start_;
 
+  // System time at which the current load was started for the purpose of
+  // the perceived page load time (PPLT).
+  base::TimeTicks pplt_load_start_;
+
   // The current load state and the URL associated with it.
   net::LoadState load_state_;
   string16 load_state_host_;