Enable MSVC warning for unused locals.

build/config/compiler/BUILD.gn

 # 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.
 
 import("//build/config/android/config.gni")
 if (cpu_arch == "arm") {
   import("//build/config/arm.gni")
 }
 if (is_posix) {
   import("//build/config/gcc/gcc_version.gni")
@@ skipping 671 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).
       "/wd4611",
 
 
       # Warnings to evaluate and possibly fix/reenable later:
 
       "/wd4100",  # Unreferenced formal function parameter.
-      "/wd4189",  # A variable was declared and initialized but never used.
       "/wd4244",  # Conversion: possible loss of data.
       "/wd4481",  # Nonstandard extension: override specifier.
       "/wd4505",  # Unreferenced local function has been removed.
       "/wd4510",  # Default constructor could not be generated.
       "/wd4512",  # Assignment operator could not be generated.
       "/wd4610",  # Class can never be instantiated, constructor required.
     ]
   } else {
     # Common GCC warning setup.
     cflags = [
@@ skipping 265 matching lines @@
       cflags += [ "-gsplit-dwarf" ]
     }
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }