clang/win: Take another stab at building with -Wunused-private-field.

build/common.gypi

 # Copyright (c) 2012 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 5714 matching lines @@
                   '/FIIntrin.h',
 
                   # TODO(hans): Make this list shorter eventually, http://crbug.com/504657
                   '-Qunused-arguments',  # http://crbug.com/504658
                   '-Wno-extra-tokens',  # http://crbug.com/504663
                   '-Wno-microsoft',  # http://crbug.com/505296
                   '-Wno-pointer-sign',  # http://crbug.com/505303
                   '-Wno-switch',  # http://crbug.com/505308
                   '-Wno-unknown-pragmas',  # http://crbug.com/505314
                   '-Wno-unused-function',  # http://crbug.com/505316
-                  '-Wno-unused-private-field',  # http://crbug.com/505317
                   '-Wno-unused-value',  # http://crbug.com/505318
                   '-Wno-unused-variable',  # http://crbug.com/505319
                   '-Wno-unused-local-typedef',  # http://crbug.com/411648
                 ],
               },
             }],
             ['clang==1 and target_arch=="ia32"', {
               'VCCLCompilerTool': {
                 'WarnAsError': 'false',
                 'AdditionalOptions': [
@@ skipping 459 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }