Revert of Enable C4018 globally for the GN build, and disable per-target.

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
Committed: https://crrev.com/4bf2ca554347fc20e1649e5ff4cf700d9fbd713a
Cr-Commit-Position: refs/heads/master@{#340623}

[ccameron, 2015-07-28 01:04:51]

Created Revert of Enable C4018 globally for the GN build, and disable
per-target.

[commit-bot: I haz the power, 2015-07-28 01:05:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1257583007/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1257583007/1

[commit-bot: I haz the power, 2015-07-28 01:06:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-28 01:07:21]

Patchset 1 (id:??) landed as
https://crrev.com/4bf2ca554347fc20e1649e5ff4cf700d9fbd713a
Cr-Commit-Position: refs/heads/master@{#340623}

[Peter Kasting, 2015-07-28 19:17:48]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1259253006/ by pkasting@chromium.org.

The reason for reverting is: I believe the GN build failure was caused by a
different patch of brettw's..