win: Stop passing /wd4206 in PCH builds.

build/config/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/allocator.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/features.gni")
@@ skipping 341 matching lines @@
       # source code in quotes for an #include directive.
       precompiled_header = "build/precompile.h"
 
       # This is a file that GN will compile with the above header. It will be
       # implicitly added to the sources (potentially multiple times, with one
       # variant for each language used in the target).
       precompiled_source = "//build/precompile.cc"
 
       # Force include the header.
       cflags = [ "/FI$precompiled_header" ]
-
-      # Disable warning for "this file was empty after preprocessing". This
-      # error is generated only in C mode for ANSI compatibility. It conflicts
-      # with precompiled headers since the source file that's "compiled" for
-      # making the precompiled header is empty.
-      #
-      # This error doesn't happen every time. In VS2013, it seems if the .pch
-      # file doesn't exist, no error will be generated (probably MS tested this
-      # case but forgot the other one?). To reproduce this error, do a build,
-      # then delete the precompile.c.obj file, then build again.
-      #
-      # TODO(sof): determine VS2015 status and retire the setting from all
-      # precompiled configurations.
-      cflags_c = [ "/wd4206" ]
     } else if (is_mac) {
       precompiled_source = "//build/precompile.h"
     }
   }
 }