Specify the path to the old setup.exe in the lpApplicationName parameter to CreateProcess.

chrome/installer/mini_installer/mini_installer.cc

 // 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.
 
 // mini_installer.exe is the first exe that is run when chrome is being
 // installed or upgraded. It is designed to be extremely small (~5KB with no
 // extra resources linked) and it has two main jobs:
 //   1) unpack the resources (possibly decompressing some)
 //   2) run the real installer (setup.exe) with appropriate flags.
 //
@@ skipping 387 matching lines @@
   // We need a chrome archive to do the installation. So if there
   // is a problem in fetching B7 resource, just return an error.
   if (!::EnumResourceNames(module, kLZMAResourceType, OnResourceFound,
                            reinterpret_cast<LONG_PTR>(&context)) ||
       archive_path->length() == 0)
     return false;
 
   // If we found setup 'B7' resource, handle it.
   if (setup_path->length() > 0) {
     CommandString cmd_line;
+    PathString exe_path;
     // Get the path to setup.exe first.
     bool success = true;
-    if (!GetSetupExePathFromRegistry(configuration, cmd_line.get(),
-                                     cmd_line.capacity()) ||
+    if (!GetSetupExePathFromRegistry(configuration, exe_path.get(),
+                                     exe_path.capacity()) ||
+        !cmd_line.append(exe_path.get()) ||
         !cmd_line.append(L" --") ||
         !cmd_line.append(kCmdUpdateSetupExe) ||
         !cmd_line.append(L"=\"") ||
         !cmd_line.append(setup_path->get()) ||
         !cmd_line.append(L"\" --") ||
         !cmd_line.append(kCmdNewSetupExe) ||
         !cmd_line.append(L"=\"") ||
         !cmd_line.append(setup_dest_path.get()) ||
         !cmd_line.append(L"\"")) {
       success = false;
     }
 
     // Get any command line option specified for mini_installer and pass them
     // on to setup.exe.  This is important since switches such as
     // --multi-install and --chrome-frame affect where setup.exe will write
     // installer results for consumption by Google Update.
     AppendCommandLineFlags(configuration, &cmd_line);
 
     ProcessExitCode exit_code = SUCCESS_EXIT_CODE;
     if (success &&
-        (!RunProcessAndWait(NULL, cmd_line.get(), &exit_code) ||
+        (!RunProcessAndWait(exe_path.get(), cmd_line.get(), &exit_code) ||
          exit_code != SUCCESS_EXIT_CODE)) {
       success = false;
     }
 
     if (!success)
       DeleteFile(setup_path->get());
 
     return success && setup_path->assign(setup_dest_path.get());
   }
 
@@ skipping 410 matching lines @@
 #pragma function(memset)
 void* memset(void* dest, int c, size_t count) {
   void* start = dest;
   while (count--) {
     *reinterpret_cast<char*>(dest) = static_cast<char>(c);
     dest = reinterpret_cast<char*>(dest) + 1;
   }
   return start;
 }
 }  // extern "C"