Add the concept of a source set to GN.

tools/gn/target_generator.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/target_generator.h"
 
 #include "tools/gn/binary_target_generator.h"
 #include "tools/gn/build_settings.h"
 #include "tools/gn/config.h"
 #include "tools/gn/copy_target_generator.h"
@@ skipping 82 matching lines @@
     BinaryTargetGenerator generator(target, scope, function_token,
                                     Target::EXECUTABLE, err);
     generator.Run();
   } else if (output_type == functions::kGroup) {
     GroupTargetGenerator generator(target, scope, function_token, err);
     generator.Run();
   } else if (output_type == functions::kSharedLibrary) {
     BinaryTargetGenerator generator(target, scope, function_token,
                                     Target::SHARED_LIBRARY, err);
     generator.Run();
+  } else if (output_type == functions::kSourceSet) {
+    BinaryTargetGenerator generator(target, scope, function_token,
+                                    Target::SOURCE_SET, err);
+    generator.Run();
   } else if (output_type == functions::kStaticLibrary) {
     BinaryTargetGenerator generator(target, scope, function_token,
                                     Target::STATIC_LIBRARY, err);
     generator.Run();
   } else {
     *err = Err(function_token, "Not a known output type",
                "I am very confused.");
   }
 }
 
@@ skipping 195 matching lines @@
     forward_from_list.push_back(forward_from);
   }
 
   target_->forward_dependent_configs().swap(forward_from_list);
 }