Add support for data deps.

tools/gn/command_gen.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 "base/atomicops.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/time/time.h"
 #include "tools/gn/build_settings.h"
@@ skipping 40 matching lines @@
   // the writing to maintain a counter.
   base::subtle::Atomic32 write_counter = 0;
   setup->build_settings().set_target_resolved_callback(
       base::Bind(&TargetResolvedCallback, &write_counter));
 
   // Do the actual load. This will also write out the target ninja files.
   if (!setup->Run())
     return 1;
 
   // Write the root ninja files.
-  if (!NinjaWriter::RunAndWriteFiles(&setup->build_settings())) {
-    Err(Location(),
-        "Couldn't open root buildfile(s) for writing").PrintToStdout();
+  if (!NinjaWriter::RunAndWriteFiles(&setup->build_settings()))
     return 1;
-  }
 
   base::TimeTicks end_time = base::TimeTicks::Now();
 
   if (!CommandLine::ForCurrentProcess()->HasSwitch(kSwitchQuiet)) {
     OutputString("Done. ", DECORATION_GREEN);
 
     std::string stats = "Wrote " +
         base::IntToString(static_cast<int>(write_counter)) +
         " targets from " +
         base::IntToString(
             setup->scheduler().input_file_manager()->GetInputFileCount()) +
         " files in " +
         base::IntToString((end_time - begin_time).InMilliseconds()) + "ms\n";
     OutputString(stats);
   }
 
   return 0;
 }
 
 }  // namespace commands