Add runtime dependency extraction for GN.

tools/gn/command_gen.cc

Index: tools/gn/command_gen.cc
diff --git a/tools/gn/command_gen.cc b/tools/gn/command_gen.cc
index 5c0a5a94f7ae2664cd2042085adf299c66e58f47..76f1f36f7c4ee841a692db6f812416025bb9a7c7 100644
--- a/tools/gn/command_gen.cc
+++ b/tools/gn/command_gen.cc
@@ -11,6 +11,7 @@
 #include "tools/gn/commands.h"
 #include "tools/gn/ninja_target_writer.h"
 #include "tools/gn/ninja_writer.h"
+#include "tools/gn/runtime_deps.h"
 #include "tools/gn/scheduler.h"
 #include "tools/gn/setup.h"
 #include "tools/gn/standard_out.h"
@@ -101,6 +102,11 @@ int RunGen(const std::vector<std::string>& args) {
     return 1;
   }
 
+  if (!WriteRuntimeDepsFilesIfNecessary(*setup->builder(), &err)) {
+    err.PrintToStdout();
+    return 1;
+  }
+
   base::TimeDelta elapsed_time = timer.Elapsed();
 
   if (!base::CommandLine::ForCurrentProcess()->HasSwitch(switches::kQuiet)) {