GN: Add optional default_args variable to .gn.

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"
 
 const char kBuildArgs_Help[] =
     R"(Build Arguments Overview
 
   Build arguments are variables passed in from outside of the build that build
   files can query to determine how the build works.
 
 How build arguments are set
 
   First, system default arguments are set based on the current system. The
   built-in arguments are:
    - host_cpu
    - host_os
    - current_cpu
    - current_os
    - target_cpu
    - target_os
 
+  Next, project-specific overrides are applied. These are specified inside
+  the default_args variable of //.gn. See "gn help dotfile" for more.
+
   If specified, arguments from the --args command line flag are used. If that
   flag is not specified, args from previous builds in the build directory will
   be used (this is in the file args.gn in the build directory).
 
   Last, for targets being compiled with a non-default toolchain, the toolchain
   overrides are applied. These are specified in the toolchain_args section of a
   toolchain definition. The use-case for this is that a toolchain may be
   building code for a different platform, and that it may want to always
   specify Posix, for example. See "gn help toolchain" for more.
 
@@ skipping 332 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()];
 }