Fix the output of "gn args" for the OS and CPU architecture.

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 "build/build_config.h"
 #include "tools/gn/variables.h"
 
 #if defined(OS_WIN)
@@ skipping 37 matching lines @@
     "  listed in the \"How build arguments are set\" section above apply to\n"
     "  the given argument, but the defaults will not override any of these.\n"
     "\n"
     "  Often, the root build config file will declare global arguments that\n"
     "  will be passed to all buildfiles. Individual build files can also\n"
     "  specify arguments that apply only to those files. It is also usedful\n"
     "  to specify build args in an \"import\"-ed file if you want such\n"
     "  arguments to apply to multiple buildfiles.\n";
 
 Args::Args() {
-  // These system values are always overridden and won't appear in a
-  // declare_args call, so mark them as used to prevent a warning later.
-  declared_arguments_[variables::kOs] = Value();
-  declared_arguments_[variables::kCpuArch] = Value();
 }
 
 Args::Args(const Args& other)
     : overrides_(other.overrides_),
       all_overrides_(other.all_overrides_),
       declared_arguments_(other.declared_arguments_) {
 }
 
 Args::~Args() {
 }
@@ skipping 99 matching lines @@
 #elif defined(OS_MACOSX)
   os = "mac";
 #elif defined(OS_LINUX)
   os = "linux";
 #else
   #error Unknown OS type.
 #endif
   Value os_val(NULL, std::string(os));
   dest->SetValue(variables::kBuildOs, os_val, NULL);
   dest->SetValue(variables::kOs, os_val, NULL);
-  declared_arguments_[variables::kBuildOs] = os_val;
-  declared_arguments_[variables::kOs] = os_val;
 
   // Host architecture.
   static const char kIa32[] = "ia32";
   static const char kIa64[] = "ia64";
   const char* arch = NULL;
 #if defined(OS_WIN)
   // ...on Windows, set the CPU architecture based on the underlying OS, not
   // whatever the current bit-tedness of the GN binary is.
   const base::win::OSInfo* os_info = base::win::OSInfo::GetInstance();
   switch (os_info->architecture()) {
@@ skipping 21 matching lines @@
     #error Unknown architecture.
   #endif
 #endif
   // Avoid unused var warning.
   (void)kIa32;
   (void)kIa64;
 
   Value arch_val(NULL, std::string(arch));
   dest->SetValue(variables::kBuildCpuArch, arch_val, NULL);
   dest->SetValue(variables::kCpuArch, arch_val, NULL);
-  declared_arguments_[variables::kBuildOs] = arch_val;
-  declared_arguments_[variables::kOs] = arch_val;
 
+  // Save the OS and architecture as build arguments that are implicitly
+  // declared. This is so they can be overridden in a toolchain build args
+  // override, and so that they will appear in the "gn args" output.
+  //
+  // Do not declare the build* variants since these shouldn't be changed.
+  //
   // Mark these variables used so the build config file can override them
   // without geting a warning about overwriting an unused variable.
+  declared_arguments_[variables::kOs] = os_val;
+  declared_arguments_[variables::kCpuArch] = arch_val;
   dest->MarkUsed(variables::kCpuArch);
   dest->MarkUsed(variables::kOs);
 }
 
 
 void Args::ApplyOverrides(const Scope::KeyValueMap& values,
                           Scope* scope) const {
   for (Scope::KeyValueMap::const_iterator i = values.begin();
        i != values.end(); ++i)
     scope->SetValue(i->first, i->second, i->second.origin());
 }
 
 void Args::SaveOverrideRecord(const Scope::KeyValueMap& values) const {
   base::AutoLock lock(lock_);
   for (Scope::KeyValueMap::const_iterator i = values.begin();
        i != values.end(); ++i)
     all_overrides_[i->first] = i->second;
 }