Add histograms to the rendez-vous process (ProcessSingleton).

chrome/browser/process_singleton.h

 // Copyright (c) 2012 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.
 
 #ifndef CHROME_BROWSER_PROCESS_SINGLETON_H_
 #define CHROME_BROWSER_PROCESS_SINGLETON_H_
 
 #include "build/build_config.h"
 
 #if defined(OS_WIN)
@@ skipping 31 matching lines @@
 // each other.  It is named according to the user data directory, so
 // we can be sure that no more than one copy of the application can be
 // running at once with a given data directory.
 //
 // Implementation notes:
 // - the Windows implementation uses an invisible global message window;
 // - the Linux implementation uses a Unix domain socket in the user data dir.
 
 class ProcessSingleton : public base::NonThreadSafe {
  public:
+  // Logged as histograms, do not modify these values.
   enum NotifyResult {
-    PROCESS_NONE,
-    PROCESS_NOTIFIED,
-    PROFILE_IN_USE,
-    LOCK_ERROR,
+    PROCESS_NONE = 0,
+    PROCESS_NOTIFIED = 1,
+    PROFILE_IN_USE = 2,
+    LOCK_ERROR = 3,
+    LAST_VALUE = LOCK_ERROR
   };
 
+  static constexpr int kNumNotifyResults = LAST_VALUE + 1;
+
   // Implement this callback to handle notifications from other processes. The
   // callback will receive the command line and directory with which the other
   // Chrome process was launched. Return true if the command line will be
   // handled within the current browser instance or false if the remote process
   // should handle it (i.e., because the current process is shutting down).
   using NotificationCallback =
       base::Callback<bool(const base::CommandLine& command_line,
                           const base::FilePath& current_directory)>;
 
   ProcessSingleton(const base::FilePath& user_data_dir,
@@ skipping 105 matching lines @@
   // Helper class for linux specific messages.  LinuxWatcher is ref counted
   // because it posts messages between threads.
   class LinuxWatcher;
   scoped_refptr<LinuxWatcher> watcher_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(ProcessSingleton);
 };
 
 #endif  // CHROME_BROWSER_PROCESS_SINGLETON_H_