Introduce MinidumpStringWriter and its test

minidump/minidump.gyp

 # Copyright 2014 The Crashpad Authors. All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
 # You may obtain a copy of the License at
 #
 #     http://www.apache.org/licenses/LICENSE-2.0
 #
 # Unless required by applicable law or agreed to in writing, software
 # distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 11 matching lines @@
         '../third_party/mini_chromium/mini_chromium/base/base.gyp:base',
         '../util/util.gyp:util',
       ],
       'export_dependent_settings': [
         '../compat/compat.gyp:compat',
       ],
       'include_dirs': [
         '..',
       ],
       'sources': [
+        'minidump_string_writer.cc',
+        'minidump_string_writer.h',
         'minidump_writable.cc',
         'minidump_writable.h',
+        'minidump_writer_util.cc',
+        'minidump_writer_util.h',
       ],
     },
     {
       'target_name': 'minidump_test',
       'type': 'executable',
       'dependencies': [
         'minidump',
         '../third_party/gtest/gtest.gyp:gtest',
         '../third_party/mini_chromium/mini_chromium/base/base.gyp:base',
       ],
       'include_dirs': [
         '..',
       ],
       'sources': [
         '../third_party/gtest/gtest/src/gtest_main.cc',
+        'minidump_string_writer_test.cc',
         'minidump_writable_test.cc',
       ],
     },
   ],
 }