Support --delete-old-versions in setup.exe.

chrome/installer/setup/setup_singleton.h

 // Copyright 2016 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_INSTALLER_SETUP_SETUP_SINGLETON_H_
 #define CHROME_INSTALLER_SETUP_SETUP_SINGLETON_H_
 
 #include <windows.h>
 
 #include <memory>
@@ skipping 31 matching lines @@
       InstallationState* original_state,
       InstallerState* installer_state);
 
   // Releases the exclusive right to modify the Chrome installation.
   ~SetupSingleton();
 
   // Waits until |max_time| has passed or another process tries to acquire a
   // SetupSingleton for the same Chrome installation. In the latter case, the
   // method returns true and this SetupSingleton should be released as soon as
   // possible to unblock the other process.
-  bool WaitForInterrupt(const base::TimeDelta& max_time);
+  bool WaitForInterrupt(const base::TimeDelta& max_time) const;
 
  private:
   class ScopedHoldMutex {
    public:
     ScopedHoldMutex();
     ~ScopedHoldMutex();
 
     // Waits up to a certain amount of time to acquire |mutex|. Returns true on
     // success. |mutex| will be released in the destructor.
     bool Acquire(HANDLE mutex);
 
    private:
     HANDLE mutex_ = INVALID_HANDLE_VALUE;
 
     DISALLOW_COPY_AND_ASSIGN(ScopedHoldMutex);
   };
 
   SetupSingleton(base::win::ScopedHandle setup_mutex,
                  base::win::ScopedHandle exit_event);
 
   // A mutex that must be held to modify the Chrome installation directory.
   base::win::ScopedHandle setup_mutex_;
 
   // Holds |setup_mutex_| throughout the lifetime of this SetupSingleton.
   ScopedHoldMutex scoped_hold_setup_mutex_;
 
   // An event signaled to ask the owner of |setup_mutex_| to release it as soon
   // as possible.
-  base::WaitableEvent exit_event_;
+  mutable base::WaitableEvent exit_event_;
 
   DISALLOW_COPY_AND_ASSIGN(SetupSingleton);
 };
 
 }  // namespace installer
 
 #endif  // CHROME_INSTALLER_SETUP_SETUP_SINGLETON_H_