GN: Add optional default_args variable to .gn.

tools/gn/setup.cc

Index: tools/gn/setup.cc
diff --git a/tools/gn/setup.cc b/tools/gn/setup.cc
index 8a433bcdbb1cacf67fd0e96bdbaf5f5bee9dc360..ed6ef54d174615bf0e14271814b0751c97fb50b5 100644
--- a/tools/gn/setup.cc
+++ b/tools/gn/setup.cc
@@ -106,6 +106,15 @@ Variables
 
       The secondary source root must be inside the main source tree.
 
+  default_args [optional]
+      Scope containing the default overrides for declared arguments. These
+      overrides take precedence over the default values specified in the
+      declare_args() block, but can be overriden using --args or the
+      args.gn file.
+
+      This is intended to be used when subprojects declare arguments with
+      default values that need to be changed for whatever reason.
+
 Example .gn file contents
 
   buildconfig = "//build/config/BUILDCONFIG.gn"
@@ -118,6 +127,12 @@ Example .gn file contents
   root = "//:root"
 
   secondary_source = "//build/config/temporary_buildfiles/"
+
+  default_args = {
+    # Default to release builds for this project.
+    is_debug = false
+    is_component_build = false
+  }
 )";
 
 namespace {
@@ -257,6 +272,7 @@ Setup::Setup()
       check_public_headers_(false),
       dotfile_settings_(&build_settings_, std::string()),
       dotfile_scope_(&dotfile_settings_),
+      default_args_(nullptr),
       fill_arguments_(true) {
   dotfile_settings_.set_toolchain_label(Label());
 
@@ -300,6 +316,14 @@ bool Setup::DoSetup(const std::string& build_dir, bool force_create) {
     return false;
   }
 
+  // Apply project-specific default (if specified).
+  // Must happen before FillArguments().
+  if (default_args_) {
+    Scope::KeyValueMap overrides;
+    default_args_->GetCurrentScopeValues(&overrides);
+    build_settings_.build_args().AddArgOverrides(overrides);
+  }
+
   if (fill_arguments_) {
     if (!FillArguments(*cmdline))
       return false;
@@ -738,5 +762,17 @@ bool Setup::FillOtherConfig(const base::CommandLine& cmdline) {
     build_settings_.set_exec_script_whitelist(std::move(whitelist));
   }
 
+  // Fill optional default_args.
+  const Value* default_args_value =
+      dotfile_scope_.GetValue("default_args", true);
+  if (default_args_value) {
+    if (!default_args_value->VerifyTypeIs(Value::SCOPE, &err)) {
+      err.PrintToStdout();
+      return false;
+    }
+
+    default_args_ = default_args_value->scope_value();
+  }
+
   return true;
 }