Add the concept of a source set to GN.

tools/gn/ninja_helper.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_helper.h"
 
 #include "base/logging.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/string_utils.h"
 #include "tools/gn/target.h"
@@ skipping 114 matching lines @@
   if (!target->settings()->IsWin() &&
       (target->output_type() == Target::SHARED_LIBRARY ||
        target->output_type() == Target::STATIC_LIBRARY) &&
       name.compare(0, 3, "lib") != 0)
     prefix = "lib";
   else
     prefix = "";
 
   const char* extension;
   if (target->output_type() == Target::GROUP ||
+      target->output_type() == Target::SOURCE_SET ||
       target->output_type() == Target::COPY_FILES ||
       target->output_type() == Target::CUSTOM) {
     extension = "stamp";
   } else {
     extension = GetExtensionForOutputType(target->output_type(),
                                           target->settings()->target_os());
   }
 
   // Everything goes into the toolchain directory (which will be empty for the
   // default toolchain, and will end in a slash otherwise).
@@ skipping 78 matching lines @@
   if (settings->IsWin()) {
     if (type == SOURCE_RC)
       return prefix + "rc";
   } else {
     if (type == SOURCE_S)
       return prefix + "cc";  // Assembly files just get compiled by CC.
   }
 
   return std::string();
 }