win: Stop passing /wd4206 in PCH builds.

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index 593a88db41dc14f0b88492fee36d1628109f1415..4a341784a2abaa6cc4b6e1301041ecbd6cadf5ed 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -359,20 +359,6 @@ config("precompiled_headers") {
 
       # 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"
     }