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

chrome/browser/process_singleton.h

Index: chrome/browser/process_singleton.h
diff --git a/chrome/browser/process_singleton.h b/chrome/browser/process_singleton.h
index fe11a1905fd5369e5acab6a1d4992e58627cf44d..96a9bf65f42d2cfab301a10573b25739e7e05a4e 100644
--- a/chrome/browser/process_singleton.h
+++ b/chrome/browser/process_singleton.h
@@ -49,13 +49,17 @@ class CommandLine;
 
 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