Revert of Enable C4018 globally for the GN build, and disable per-target. (patchset #4 id:60001 of https://codereview.chromium.org/1246103007/)

Reason for revert:
Speculative revert based on windows gn bots failing with

http://build.chromium.org/p/chromium.win/builders/Win8%20GN%20%28dbg%29/builds/10113

FAILED: ninja -t msvc -e environment.x64 -- E:\b\build\goma/gomacc.exe "E:\b\depot_tools\win_toolchain\vs2013_files\VC\bin\amd64/cl.exe" /nologo /showIncludes /FC @obj/base/base/precompile.c.obj.rsp /c ../../build/precompile.cc /Foobj/base/base/precompile.c.obj /Fdobj/base/base_c.pdb
e:\b\build\slave\win_x64_gn__dbg_\build\src\build\precompile.cc(8) : error C2220: warning treated as error - no 'object' file generated
e:\b\build\slave\win_x64_gn__dbg_\build\src\build\precompile.cc(8) : warning C4206: nonstandard extension used : translation unit is empty
ninja: build stopped: subcommand failed.

In theory this warning shouldn't have affected other warnings, but there are no other clear candidates.

Original issue's description:
> Enable C4018 globally for the GN build, and disable per-target.
>
> This matches what the GYP build does.
>
> BUG=398202
> TEST=none
>
> Committed: https://crrev.com/d408e1965f8e03629738039289ab7779c42a318c
> Cr-Commit-Position: refs/heads/master@{#340590}

TBR=brettw@chromium.org,erg@chromium.org,pkasting@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=398202

Review URL: https://codereview.chromium.org/1257583007

Cr-Original-Commit-Position: refs/heads/master@{#340623}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4bf2ca554347fc20e1649e5ff4cf700d9fbd713a

config/compiler/BUILD.gn

@@ -717,6 +717,11 @@ if (is_win) {
 
     # Warnings permanently disabled:
 
+    # TODO(GYP) The GYP build doesn't have this globally enabled but disabled
+    # for a bunch of individual targets. Re-enable this globally when those
+    # targets are fixed.
+    "/wd4018",  # Comparing signed and unsigned values.
+
     # C4127: conditional expression is constant
     # This warning can in theory catch dead code and other problems, but
     # triggers in far too many desirable cases where the conditional