Add support for a 'default' target to GN.

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 <map>
 
 #include "base/command_line.h"
@@ skipping 238 matching lines @@
   // Pick up phony rules for the toplevel targets with non-unique names (which
   // would have been skipped in the above loop).
   for (size_t i = 0; i < toplevel_targets.size(); i++) {
     if (small_name_count[toplevel_targets[i]->label().name()] > 1) {
       const Target* target = toplevel_targets[i];
       WritePhonyRule(target, target->dependency_output_file(),
                      target->label().name());
     }
   }
 
+  // Figure out if the BUILD file wants to declare a custom "default"
+  // target (rather than building 'all' by default). By convention
+  // we use group("default") but it doesn't have to be a group.
+  bool default_target_exists = false;
+  for (size_t i = 0; i < default_toolchain_targets_.size(); i++) {
+    const Label& label = default_toolchain_targets_[i]->label();
+    if (label.dir().value() == "//" && label.name() == "default")
+      default_target_exists = true;
+  }
+
   if (!all_rules.empty()) {
     out_ << "\nbuild all: phony " << all_rules << std::endl;
+  }
+
+  if (default_target_exists) {
+    out_ << "default default" << std::endl;
+  } else if (!all_rules.empty()) {
     out_ << "default all" << std::endl;
   }
+
   return true;
 }
 
 void NinjaBuildWriter::WritePhonyRule(const Target* target,
                                       const OutputFile& target_file,
                                       const std::string& phony_name) {
   if (target_file.value() == phony_name)
     return;  // No need for a phony rule.
 
   EscapeOptions ninja_escape;
   ninja_escape.mode = ESCAPE_NINJA;
 
   // Escape for special chars Ninja will handle.
   std::string escaped = EscapeString(phony_name, ninja_escape, NULL);
 
   out_ << "build " << escaped << ": phony ";
   path_output_.WriteFile(out_, target_file);
   out_ << std::endl;
 }