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

chrome/installer/mini_installer/BUILD.gn

 # Copyright 2015 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.
 
 import("//build/config/compiler/compiler.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//chrome/process_version_rc_template.gni")
 import("//chrome/test/mini_installer/mini_installer_test.gni")
 import("//third_party/icu/config.gni")
 import("//ui/base/ui_features.gni")
 import("//v8/gni/v8.gni")
 
 config("mini_installer_compiler_flags") {
   # Disable buffer security checking.
   cflags = [ "/GS-" ]
 }
 
 source_set("lib") {
   sources = [
     "appid.h",
     "chrome.release",
     "chrome_appid.cc",
     "configuration.cc",
     "configuration.h",
     "decompress.cc",
     "decompress.h",
+    "mini_installer.cc",
+    "mini_installer.h",
     "mini_installer.ico",
     "mini_installer.rc",
     "mini_installer_constants.cc",
     "mini_installer_constants.h",
     "mini_installer_resource.h",
     "mini_string.cc",
     "mini_string.h",
     "pe_resource.cc",
     "pe_resource.h",
     "regkey.cc",
@@ skipping 173 matching lines @@
     depfile = "$target_gen_dir/archive.d"
     args += [
       "--depfile",
       rebase_path(depfile, root_build_dir),
     ]
   }
 
   executable(target_name) {
     output_name = "mini_installer"
     sources = [
-      "mini_installer.cc",
+      "mini_installer_exe_main.cc",
       packed_files_rc_file,
     ]
 
     # This target is special so we manually override most linker flags and
     # specify our own to keep the size down. Also make sure that we don't use
     # WPO as it's not supported by the mini installer.
     configs -= [
       "//build/config/compiler:default_optimization",
       "//build/config:executable_config",
       "//build/config/win:console",
@@ skipping 58 matching lines @@
     args = [
       "--out",
       "$target_name.exe",
     ]
     deps = [
       ":mini_installer",
       "//chrome/installer/test:alternate_version_generator",
     ]
   }
 }