[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..80aa0581fa3bcc10be450424d846b25a3ef716e1 100644
--- a/chrome/browser/sessions/tab_loader.cc
+++ b/chrome/browser/sessions/tab_loader.cc
@@ -103,6 +103,8 @@ TabLoader::TabLoader(base::TimeTicks restore_started)
   shared_tab_loader_ = this;
   this_retainer_ = this;
   base::MemoryCoordinatorClientRegistry::GetInstance()->Register(this);
+  for (auto& observer : SessionRestore::observers())
+    observer.OnSessionRestoreStartedLoadingTabs();
 }
 
 TabLoader::~TabLoader() {
@@ -110,6 +112,8 @@ TabLoader::~TabLoader() {
   DCHECK(shared_tab_loader_ == this);
   shared_tab_loader_ = nullptr;
   base::MemoryCoordinatorClientRegistry::GetInstance()->Unregister(this);
+  for (auto& observer : SessionRestore::observers())
+    observer.OnSessionRestoreFinishedLoadingTabs();
 }
 
 void TabLoader::StartLoading(const std::vector<RestoredTab>& tabs) {