Fix MSVC build (including on 2015), drop some workarounds for MSVC older than 2013.

src/client/windows/unittests/client_tests.gyp

 # Copyright 2010 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
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@@ skipping 55 matching lines @@
         ]
       },
       'sources': [
         '<(DEPTH)/common/string_conversion.cc',
         '<(DEPTH)/processor/basic_code_modules.cc',
         '<(DEPTH)/processor/dump_context.cc',
         '<(DEPTH)/processor/dump_object.cc',
         '<(DEPTH)/processor/logging.cc',
         '<(DEPTH)/processor/minidump.cc',
         '<(DEPTH)/processor/pathname_stripper.cc',
+        '<(DEPTH)/processor/proc_maps_linux.cc',
       ]
     }
   ],
 }