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

tools/gn/ninja_build_writer.cc

Index: tools/gn/ninja_build_writer.cc
diff --git a/tools/gn/ninja_build_writer.cc b/tools/gn/ninja_build_writer.cc
index b7cfdd797cdb620617fc8dd5a8ed9d393a7d44f0..a7752453ec27d9bb8bdbd72acf19bf4520cb14d1 100644
--- a/tools/gn/ninja_build_writer.cc
+++ b/tools/gn/ninja_build_writer.cc
@@ -136,9 +136,26 @@ void NinjaBuildWriter::WriteNinjaRules() {
   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+  // something is out-of-date.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,