gn format: support absolute file system paths

tools/gn/source_file.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/source_file.h"
 
 #include "base/logging.h"
 #include "build/build_config.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/source_dir.h"
@@ skipping 46 matching lines @@
     if (value_.size() > 2 && value_[2] == ':') {
       // Windows path, strip the leading slash.
       converted.assign(&value_[1], value_.size() - 1);
     } else {
       converted.assign(value_);
     }
     return base::FilePath(UTF8ToFilePath(converted));
   }
 
   converted.assign(&value_[2], value_.size() - 2);
+  if (source_root.empty())
+    return UTF8ToFilePath(converted).NormalizePathSeparatorsTo('/');
   return source_root.Append(UTF8ToFilePath(converted))
       .NormalizePathSeparatorsTo('/');
 }