Redo GN "args" command

tools/gn/setup.h

Index: tools/gn/setup.h
diff --git a/tools/gn/setup.h b/tools/gn/setup.h
index eb9d37b2b445288700335c0b6e3af8fdacdd1d3b..0cd0a7930a1cf6a1ab9b2e93b4041ec0e1aa8dc6 100644
--- a/tools/gn/setup.h
+++ b/tools/gn/setup.h
@@ -58,6 +58,10 @@ class CommonSetup {
   Builder* builder() { return builder_.get(); }
   LoaderImpl* loader() { return loader_.get(); }
 
+  // Name of the file in the root build directory that contains the build
+  // arguements.
+  static const char kBuildArgFileName[];
+
  protected:
   CommonSetup();
   CommonSetup(const CommonSetup& other);
@@ -105,10 +109,31 @@ class Setup : public CommonSetup {
 
   virtual Scheduler* GetScheduler() OVERRIDE;
 
+  // Returns the file used to store the build arguments. Note that the path
+  // might not exist.
+  SourceFile GetBuildArgFile() const;
+
+  // Sets whether the build arguments should be filled during setup from the
+  // command line/build argument file. This will be true by default. The use
+  // case for setting it to false is when editing build arguments, we don't
+  // want to rely on them being valid.
+  void set_fill_arguments(bool fa) { fill_arguments_ = fa; }
+
  private:
   // Fills build arguments. Returns true on success.
   bool FillArguments(const base::CommandLine& cmdline);
 
+  // Fills the build arguments from the command line or from the build arg file.
+  bool FillArgsFromCommandLine(const std::string& args);
+  bool FillArgsFromFile();
+
+  // Given an already-loaded args_input_file_, parses and saves the resulting
+  // arguments. Backend for the different FillArgs variants.
+  bool FillArgsFromArgsInputFile();
+
+  // Writes the build arguments to the build arg file.
+  bool SaveArgsToFile();
+
   // Fills the root directory into the settings. Returns true on success.
   bool FillSourceDir(const base::CommandLine& cmdline);
 
@@ -140,6 +165,10 @@ class Setup : public CommonSetup {
   std::vector<Token> dotfile_tokens_;
   scoped_ptr<ParseNode> dotfile_root_;
 
+  // Set to true when we should populate the build arguments from the command
+  // line or build argument file. See setter above.
+  bool fill_arguments_;
+
   // State for invoking the command line args. We specifically want to keep
   // this around for the entire run so that Values can blame to the command
   // line when we issue errors about them.