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

chrome/browser/resource_coordinator/tab_manager.cc

Index: chrome/browser/resource_coordinator/tab_manager.cc
diff --git a/chrome/browser/resource_coordinator/tab_manager.cc b/chrome/browser/resource_coordinator/tab_manager.cc
index 9ce446cfd721ac2a23a6c8128224e1b70016d080..c3080fa1a1e5f93df1161cefc752203fb4e75052 100644
--- a/chrome/browser/resource_coordinator/tab_manager.cc
+++ b/chrome/browser/resource_coordinator/tab_manager.cc
@@ -136,6 +136,7 @@ TabManager::TabManager()
 #endif
       browser_tab_strip_tracker_(this, nullptr, nullptr),
       test_tick_clock_(nullptr),
+      in_session_restore_(false),
       weak_ptr_factory_(this) {
 #if defined(OS_CHROMEOS)
   delegate_.reset(new TabManagerDelegate(weak_ptr_factory_.GetWeakPtr()));
@@ -475,6 +476,16 @@ int64_t TabManager::IdFromWebContents(WebContents* web_contents) {
   return reinterpret_cast<int64_t>(web_contents);
 }
 
+void TabManager::OnSessionRestoreStarted() {
+  DCHECK(!in_session_restore_);
+  in_session_restore_ = true;
+}
+
+void TabManager::OnSessionRestoreEnded() {
+  DCHECK(in_session_restore_);
+  in_session_restore_ = false;
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 // TabManager, private: