Add support for module-definition files to gn

tools/gn/source_file_type.cc

 // Copyright 2014 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/source_file_type.h"
 
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/source_file.h"
 
 SourceFileType GetSourceFileType(const SourceFile& file) {
   base::StringPiece extension = FindExtension(&file.value());
   if (extension == "cc" || extension == "cpp" || extension == "cxx")
     return SOURCE_CC;
   if (extension == "h")
     return SOURCE_H;
   if (extension == "c")
     return SOURCE_C;
   if (extension == "m")
     return SOURCE_M;
   if (extension == "mm")
     return SOURCE_MM;
   if (extension == "rc")
     return SOURCE_RC;
   if (extension == "S" || extension == "s" || extension == "asm")
     return SOURCE_S;
   if (extension == "o" || extension == "obj")
     return SOURCE_O;
+  if (extension == "def")
+    return SOURCE_DEF;
 
   return SOURCE_UNKNOWN;
 }