Add the concept of a source set to GN.

tools/gn/functions.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/functions.h"
 
 #include <iostream>
 
 #include "base/strings/string_util.h"
 #include "tools/gn/config.h"
@@ skipping 549 matching lines @@
     INSERT_FUNCTION(Group)
     INSERT_FUNCTION(Import)
     INSERT_FUNCTION(Print)
     INSERT_FUNCTION(ProcessFileTemplate)
     INSERT_FUNCTION(ReadFile)
     INSERT_FUNCTION(RebasePath)
     INSERT_FUNCTION(SetDefaults)
     INSERT_FUNCTION(SetDefaultToolchain)
     INSERT_FUNCTION(SetSourcesAssignmentFilter)
     INSERT_FUNCTION(SharedLibrary)
+    INSERT_FUNCTION(SourceSet)
     INSERT_FUNCTION(StaticLibrary)
     INSERT_FUNCTION(Template)
     INSERT_FUNCTION(Test)
     INSERT_FUNCTION(Tool)
     INSERT_FUNCTION(Toolchain)
     INSERT_FUNCTION(ToolchainArgs)
     INSERT_FUNCTION(WriteFile)
 
     #undef INSERT_FUNCTION
   }
@@ skipping 62 matching lines @@
     return found_function->second.executed_block_runner(
         function, args.list_value(), &block_scope, err);
   }
 
   // Otherwise it's a no-block function.
   return found_function->second.no_block_runner(scope, function,
                                                 args.list_value(), err);
 }
 
 }  // namespace functions