Add information on the requirements for a new Chromium port.

tools/gn/args.cc

 // Copyright (c) 2013 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.
 
 #include "tools/gn/args.h"
 
 #include "base/sys_info.h"
 #include "build/build_config.h"
 #include "tools/gn/string_utils.h"
 #include "tools/gn/variables.h"
@@ skipping 289 matching lines @@
   os = "linux";
 #elif defined(OS_ANDROID)
   os = "android";
 #elif defined(OS_NETBSD)
   os = "netbsd";
 #elif defined(OS_AIX)
   os = "aix";
 #else
   #error Unknown OS type.
 #endif
+  // NOTE: Adding a new port? Please follow
+  // https://chromium.googlesource.com/chromium/src/+/master/docs/new_port_policy.md
 
   // Host architecture.
   static const char kX86[] = "x86";
   static const char kX64[] = "x64";
   static const char kArm[] = "arm";
   static const char kArm64[] = "arm64";
   static const char kMips[] = "mipsel";
   static const char kS390X[] = "s390x";
   static const char kPPC64[] = "ppc64";
   const char* arch = nullptr;
@@ skipping 84 matching lines @@
     Scope* scope) const {
   lock_.AssertAcquired();
   return declared_arguments_per_toolchain_[scope->settings()];
 }
 
 Scope::KeyValueMap& Args::OverridesForToolchainLocked(
     Scope* scope) const {
   lock_.AssertAcquired();
   return toolchain_overrides_[scope->settings()];
 }