Clang on Windows: Remove -Wno-unused-variable.

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 3093 matching lines @@
         # clang is rolled far enough that the pinned clang understands this flag
         # TODO(thakis): Enable this, crbug.com/507717
         'variables': { 'clang_warning_flags': ['-Wno-shift-negative-value']},
       }],
       ['chromium_code==0', {
         'variables': {
           'clang_warning_flags': [
             # TODO(mgiuca): Move this suppression into individual third-party
             # libraries as required. http://crbug.com/505301.
             '-Wno-overloaded-virtual',
+            # Lots of third-party libraries have unused variables. Instead of
+            # suppressing them individually, we just blanket suppress them here.
+            '-Wno-unused-variable',
           ],
         },
         'conditions': [
           [ 'os_posix==1 and OS!="mac" and OS!="ios"', {
             # We don't want to get warnings from third-party code,
             # so remove any existing warning-enabling flags like -Wall.
             'cflags!': [
               '-Wall',
               '-Wextra',
             ],
@@ skipping 2604 matching lines @@
                   '-Qunused-arguments',  # http://crbug.com/504658
                   '-Wno-extra-tokens',  # http://crbug.com/504663
                   '-Wno-microsoft',  # http://crbug.com/505296
                   '-Wno-missing-braces',  # http://crbug.com/505297
                   '-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': [
                   '/fallback',
                 ],
@@ skipping 457 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',
   },
 }