Drop MSVC pch warning avoidance.

third_party/WebKit/Source/BUILD.gn

 # Copyright 2014 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("config.gni")
 if (is_clang) {
   import("//build/config/clang/clang.gni")
 }
 
 visibility = [ "//third_party/WebKit/*" ]
@@ skipping 51 matching lines @@
       precompiled_header = rebase_path("build/win/Precompile.h", root_build_dir)
 
       # 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 =
           "//third_party/WebKit/Source/build/win/Precompile.cpp"
 
       # 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.
-      cflags_c = [ "/wd4206" ]
     }
   }
 }
 
 # config -----------------------------------------------------------------------
 
 config("config") {
   include_dirs = [
     ".",
     "..",
@@ skipping 82 matching lines @@
   }
 }
 
 if (is_mac) {
   # This sets up precompiled headers for Mac.
   config("mac_precompiled_headers") {
     precompiled_header = rebase_path("build/mac/Prefix.h", root_build_dir)
     precompiled_source = "//third_party/WebKit/Source/build/mac/Prefix.h"
   }
 }