[TabMetrics] Add signals that mark the start and end of session restore.

chrome/browser/sessions/tab_loader.cc

Index: chrome/browser/sessions/tab_loader.cc
diff --git a/chrome/browser/sessions/tab_loader.cc b/chrome/browser/sessions/tab_loader.cc
index 753f4e0493c23ad66dc15e7f1f37a6574ce3f7e1..62e08f8865f86774efe914648340307fdff9fa4d 100644
--- a/chrome/browser/sessions/tab_loader.cc
+++ b/chrome/browser/sessions/tab_loader.cc
@@ -13,6 +13,7 @@
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram_macros.h"
 #include "build/build_config.h"
+#include "chrome/browser/sessions/session_restore_observer.h"
 #include "chrome/browser/sessions/session_restore_stats_collector.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/browser_finder.h"
@@ -103,6 +104,8 @@ TabLoader::TabLoader(base::TimeTicks restore_started)
   shared_tab_loader_ = this;
   this_retainer_ = this;
   base::MemoryCoordinatorClientRegistry::GetInstance()->Register(this);
+  for (auto& observer : SessionRestoreObserver::observers())
+    observer.OnTabLoadingStarted();
 }
 
 TabLoader::~TabLoader() {
@@ -110,6 +113,8 @@ TabLoader::~TabLoader() {
   DCHECK(shared_tab_loader_ == this);
   shared_tab_loader_ = nullptr;
   base::MemoryCoordinatorClientRegistry::GetInstance()->Unregister(this);
+  for (auto& observer : SessionRestoreObserver::observers())
+    observer.OnTabLoadingEnded();
 }
 
 void TabLoader::StartLoading(const std::vector<RestoredTab>& tabs) {