Add "ninja show" and "ninja refresh" targets to GN build.

tools/gn/ninja_build_writer.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/ninja_build_writer.h"
 
 #include <fstream>
 
 #include "base/command_line.h"
 #include "base/file_util.h"
@@ skipping 118 matching lines @@
                        default_toolchain_targets, file, depfile);
   gen.Run();
   return true;
 }
 
 void NinjaBuildWriter::WriteNinjaRules() {
   out_ << "rule gn\n";
   out_ << "  command = " << GetSelfInvocationCommand(build_settings_) << "\n";
   out_ << "  description = Regenerating ninja files\n\n";
 
+  // This rule will regenerate the ninja files when any input file has changed.
   out_ << "build build.ninja: gn\n"
        << "  depfile = build.ninja.d\n";
 
+  // Provide a way to force regenerating ninja files if the user is suspicious
+  // something is out-of-date. This will be "ninja refresh".
+  out_ << "\nbuild refresh: gn\n";
+
+  // Provide a way to see what flags are associated with this build:
+  // This will be "ninja show".
+  const CommandLine& our_cmdline = *CommandLine::ForCurrentProcess();
+  std::string args = our_cmdline.GetSwitchValueASCII("args");
+  out_ << "rule echo\n";
+  out_ << "  command = echo $text\n";
+  out_ << "  description = ECHO $desc\n";
+  out_ << "build show: echo\n";
+  out_ << "  desc = build arguments:\n";
+  out_ << "  text = " << (args.empty() ? std::string("<default args>") : args)
+       << "\n";
+
   // Input build files. These go in the ".d" file. If we write them as
   // dependencies in the .ninja file itself, ninja will expect the files to
   // exist and will error if they don't. When files are listed in a depfile,
   // missing files are ignored.
   dep_out_ << "build.ninja:";
   std::vector<base::FilePath> input_files;
   g_scheduler->input_file_manager()->GetAllPhysicalInputFileNames(&input_files);
   for (size_t i = 0; i < input_files.size(); i++)
     dep_out_ << " " << FilePathToUTF8(input_files[i]);
 
@@ skipping 34 matching lines @@
       all_rules.append(" $\n    ");
     all_rules.append(target_file.value());
   }
 
   if (!all_rules.empty()) {
     out_ << "\nbuild all: phony " << all_rules << std::endl;
     out_ << "default all" << std::endl;
   }
 }