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

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")
 import("//build/config/chrome_build.gni")
 if (current_cpu == "arm") {
   import("//build/config/arm.gni")
 }
 if (current_cpu == "mipsel" || current_cpu == "mips64el") {
@@ skipping 790 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
     ]
   }
 } else {
   # Common GCC warning setup.
   default_warning_flags += [
     # Enables.
     "-Wendif-labels",  # Weird old-style text after an #endif.
     "-Werror",  # Warnings as errors.
 
@@ skipping 176 matching lines @@
     # not support this option, since they are usually using gcc 4.2.1, which
     # does not have this flag yet.
     cflags += [ "-Wno-unused-result" ]
   }
 
   if (is_clang) {
     cflags += [
       # 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",
     ]
   }
 
   if (is_linux || is_android) {
     cflags += [
       # Don't warn about printf format problems. This is off by default in gcc
       # but on in Ubuntu's gcc(!).
       "-Wno-format",
     ]
     cflags_cc += [
@@ skipping 227 matching lines @@
       cflags += [ "-gsplit-dwarf" ]
     }
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }