chromium 添加features特性编译参数开关

具体参考gn的说明。

1，在全局里面设置编译参数的值：electron/build/args/testing.gn里面设置开关：

enable_desktop_capturer=false 会覆盖调定义时的值
也可使直接使用：
gn --args="enable_doom_melon=true enable_teleporter=false"

2，在使用它的模块中定义它，用declare_args()。

electron模块：srcelectronuildflagsuildflags.gni

# Copyright (c) 2018 GitHub, Inc.
# Use of this source code is governed by the MIT license that can be
# found in the LICENSE file.

declare_args() {
  enable_desktop_capturer = true

  # Enable Chrome extensions support.
  enable_electron_extensions = true

  # Enable Spellchecker support
  enable_builtin_spellchecker = true
}

打印模块srcprintingbuildflagsuildflags.gni

import("//build/config/chromecast_build.gni")
import("//build/config/chromeos/args.gni")
import("//build/config/features.gni")
import("//build/config/sanitizers/sanitizers.gni")

declare_args() {
  # Enable basic printing support and UI.
  enable_basic_printing = !is_chromecast && !is_ios && !is_fuchsia
}

 3，在c++中引用：

  #include "electron/buildflags/buildflags.h"

4，gn，gni文件引用它直接用 import("//printing/buildflags/buildflags.gni")

gni中：

    if (enable_print_preview) {
      sources += [ "$root_gen_dir/chrome/print_preview_resources.pak" ]
      deps += [ "//chrome/browser/resources:print_preview_resources" ]
    }

原理：

将srcprintinguildflagsuildflags.gni 

在c++中，它会生成在gen目录，ENABLE_PDF_VIEWER的头文件在 D:develectron9srcoutTestinggenelectronuildflagsuildflags.h这里。

#if BUILDFLAG(ENABLE_PRINT_PREVIEW)
  pdf::PDFWebContentsHelper::CreateForWebContentsWithClient(
      web_contents, std::make_unique<ElectronPDFWebContentsHelperClient>());
#endif

这个buildflag宏的产生需要：

D:develectron9srcoutTestinggenprintinguildflagsuildflags.h
// Generated by build/write_buildflag_header.py
// From "//printing/buildflags:buildflags"

#ifndef PRINTING_BUILDFLAGS_BUILDFLAGS_H_
#define PRINTING_BUILDFLAGS_BUILDFLAGS_H_

#include "build/buildflag.h"

#define BUILDFLAG_INTERNAL_ENABLE_PRINTING() (1)
#define BUILDFLAG_INTERNAL_ENABLE_PRINT_PREVIEW() (1)
#define BUILDFLAG_INTERNAL_ENABLE_TAGGED_PDF() (1)

#endif  // PRINTING_BUILDFLAGS_BUILDFLAGS_H_

---

pdf的一次electron提交：

SHA-1: d590f2efe3abaeda909885855c2b901345284b55

* feat: enable pdf viewer (#21794)

---