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

chrome/browser/chrome_browser_main.cc

Index: chrome/browser/chrome_browser_main.cc
diff --git a/chrome/browser/chrome_browser_main.cc b/chrome/browser/chrome_browser_main.cc
index c23936ba25af2a4ed92a0086711e7e14a9b440eb..148940205e343b8b41e30393a43b2548c938c7ab 100644
--- a/chrome/browser/chrome_browser_main.cc
+++ b/chrome/browser/chrome_browser_main.cc
@@ -1646,6 +1646,9 @@ int ChromeBrowserMainParts::PreMainMessageLoopRunImpl() {
   // 20 seconds to respond. Note that this needs to be done before we attempt
   // to read the profile.
   notify_result_ = process_singleton_->NotifyOtherProcessOrCreate();
+  UMA_HISTOGRAM_ENUMERATION("Chrome.ProcessSingleton.NotifyResult",
+                            notify_result_,
+                            ProcessSingleton::NUM_NOTIFY_RESULTS);
   switch (notify_result_) {
     case ProcessSingleton::PROCESS_NONE:
       // No process already running, fall through to starting a new one.
@@ -1679,8 +1682,9 @@ int ChromeBrowserMainParts::PreMainMessageLoopRunImpl() {
                     "now to avoid profile corruption.";
       return chrome::RESULT_CODE_PROFILE_IN_USE;
 
-    default:
+    case ProcessSingleton::NUM_NOTIFY_RESULTS:

[sky, 2016/10/07 19:47:30]

Having to delete with fake enum values like this is awkward. What about a
constant for the num results? e.g.

class ProcessSingleton {
  ...
    LAST_VALUE = LOCK_ERROR,
  ...
  constexpr kNumNotifyResults = LAST_VALUE;

[gab, 2016/10/07 20:05:46]

I like that a lot actually, done.

       NOTREACHED();
+      break;
   }
 #endif  // !defined(OS_ANDROID)