Add ExitFunnel to prepare for instrumenting browser exits.

components/browser_watcher/watcher_metrics_provider_win.h

 // Copyright (c) 2014 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 COMPONENTS_BROWSER_WATCHER_WATCHER_METRICS_PROVIDER_WIN_H_
 #define COMPONENTS_BROWSER_WATCHER_WATCHER_METRICS_PROVIDER_WIN_H_
 
 #include "base/macros.h"
 #include "base/strings/string16.h"
 #include "components/metrics/metrics_provider.h"
 
 namespace browser_watcher {
 
 // Provides stability data captured by the Chrome Watcher, namely the browser
-// process exit codes.
+// process exit codes, as well as exit funnel metrics.
+// The exit funnel records a trace of named, timed events in registry per
+// process. For reporting, the trace is recorded as a sequence of events
+// named Stability.ExitFunnel.<eventname>, associated to the time
+// (in milliseconds) from first event in a trace. For a normal process exit,
+// the sequence might look like this:
+// - Stability.ExitFunnel.Logoff: 0
+// - Stability.ExitFunnel.NotifyShutdown: 10
+// - Stability.ExitFunnel.EndSession: 20
+// - Stability.ExitFunnel.KillProcess: 30
 class WatcherMetricsProviderWin : public metrics::MetricsProvider {
  public:
   static const char kBrowserExitCodeHistogramName[];
+  static const char kExitFunnelHistogramPrefix[];
 
   explicit WatcherMetricsProviderWin(const base::char16* registry_path);
   ~WatcherMetricsProviderWin();
 
   // metrics::MetricsProvider implementation.
   virtual void ProvideStabilityMetrics(
       metrics::SystemProfileProto* system_profile_proto) override;
 
  private:
   base::string16 registry_path_;
 
   DISALLOW_COPY_AND_ASSIGN(WatcherMetricsProviderWin);
 };
 
 }  // namespace browser_watcher
 
 #endif  // COMPONENTS_BROWSER_WATCHER_WATCHER_METRICS_PROVIDER_WIN_H_