Enable MSVC warning for unused locals.

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 5386 matching lines @@
           # Chromium code compiles without exception support.  We therefore have
           # to use setjmp/longjmp for e.g. JPEG decode error handling, which
           # means we have to turn off this warning (and be careful about how
           # object destruction happens in such cases).
           4611,
 
           # TODO(maruel): These warnings are level 4. They will be slowly
           # removed as code is fixed.
           4100, # Unreferenced formal parameter
           4121, # Alignment of a member was sensitive to packing
-          4189, # Local variable is initialized but not referenced
           4244, # Conversion from 'type1' to 'type2', possible loss of data
           4481, # Nonstandard extension used: override specifier 'keyword'
           4505, # Unreferenced local function has been removed
           4510, # Default constructor could not be generated
           4512, # Assignment operator could not be generated
           4610, # Object can never be instantiated
           4996, # 'X': was declared deprecated (for GetVersionEx).
         ],
         'msvs_settings': {
           'VCCLCompilerTool': {
@@ skipping 432 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',
   },
 }