[Sessions] Add detailed logging of SessionRestore events.

content/public/browser/session_restore_uma.cc

+// Copyright 2015 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.
+//
+// UMA related constants for session restore experiments and metrics.
+
+#include "content/public/browser/session_restore_uma.h"
+
+namespace content {
+
+// Field trials.
+
+// The name of the ongoing session restore field trial.
+const char kSessionRestoreBackgroundLoading[] =
+    "SessionRestoreBackgroundLoading";
+
+// Metrics.
+
+const char kSessionRestoreActions[] = "SessionRestore.Actions";
+
+const char kSessionRestoreTabCount[] = "SessionRestore.TabCount";
+
+const char kSessionRestoreTabActions[] = "SessionRestore.TabActions";
+
+}  // namespace content