Extract the mini_installer's entrypoint into its own source file.

chrome/installer/mini_installer/mini_installer.h

+// Copyright 2017 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_MINI_INSTALLER_MINI_INSTALLER_H_
+#define CHROME_INSTALLER_MINI_INSTALLER_MINI_INSTALLER_H_
+
+#include <windows.h>
+
+#include "chrome/installer/mini_installer/exit_code.h"
+
+namespace mini_installer {
+
+// A container of a process exit code (eventually passed to ExitProcess) and
+// a Windows error code for cases where the exit code is non-zero.
+struct ProcessExitResult {
+  DWORD exit_code;
+  DWORD windows_error;
+
+  explicit ProcessExitResult(DWORD exit) : exit_code(exit), windows_error(0) {}
+  ProcessExitResult(DWORD exit, DWORD win)
+      : exit_code(exit), windows_error(win) {}
+
+  bool IsSuccess() const { return exit_code == SUCCESS_EXIT_CODE; }
+};
+
+// Main function for Chrome's mini_installer. First gets a working dir, unpacks
+// the resources, and finally executes setup.exe to do the install/update. Also
+// handles invoking a previous version's setup.exe to patch itself in the case
+// of differential updates.
+ProcessExitResult WMain(HMODULE module);
+
+}  // namespace mini_installer
+
+#endif  // CHROME_INSTALLER_MINI_INSTALLER_MINI_INSTALLER_H_