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

chrome/browser/sessions/session_restore_observer.cc

Index: chrome/browser/sessions/session_restore_observer.cc
diff --git a/chrome/browser/sessions/session_restore_observer.cc b/chrome/browser/sessions/session_restore_observer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..77d403fda700ed83bcdcefa1b46b8fc075db33e8
--- /dev/null
+++ b/chrome/browser/sessions/session_restore_observer.cc
@@ -0,0 +1,39 @@
+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/browser/sessions/session_restore_observer.h"
+
+#include <algorithm>
+
+#include "base/logging.h"
+
+// static
+std::unordered_set<SessionRestoreObserver*>*
+SessionRestoreObserver::observers() {
+  if (!observers_)
+    observers_ = new std::unordered_set<SessionRestoreObserver*>();
+  return observers_;
+}
+
+// static
+void SessionRestoreObserver::AddObserver(SessionRestoreObserver* observer) {
+  DCHECK(!observers()->count(observer)) << "Duplicate registration";
+  observers()->insert(observer);
+}
+
+// static
+void SessionRestoreObserver::RemoveObserver(SessionRestoreObserver* observer) {
+  auto found = std::find(observers()->begin(), observers()->end(), observer);
+  DCHECK(found != observers()->end());
+  observers()->erase(observer);
+
+  if (observers()->empty()) {
+    delete observers_;
+    observers_ = nullptr;
+  }
+}
+
+// static
+std::unordered_set<SessionRestoreObserver*>*
+    SessionRestoreObserver::observers_ = nullptr;