Re-disable warning 4018 in the GN Windows build.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 20a8de4133a25e31e8584476bdbe37882c6907be..456ab1958edeedcf6dd59b21420da7d2f46f8b31 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -568,6 +568,11 @@ config("default_warnings") {
 
       # Warnings permanently disabled:
 
+      # TODO(GYP) The GYP build doesn't have this globally enabled but disabled

[Peter Kasting, 2014/07/28 11:20:47]

Is "doesn't have" here supposed to read "has"?

Why are these targets (those that build in GN, at least) not also disabling this
warning in the GN build?  It concerns me if the GN build is globally disabling
something that isn't globally disabled in the GYP build.

[brettw, 2014/07/28 18:05:42]

Previously it was globally disabled in the GYP build and then a bunch of targets
individually disabled it. I don't know why this was the case and I'm quire
surprised that removing the global one actually worked.

In GYP we have a bunch of stuff disabled globally and the same things disabled
per-target. I'm not duplicating the per-target ones if it's disabled globally.

I agree that having a divergence is bad. I think the right solution is to either
disable it in GYP globally again or go in and add it to the gn files missing it.

[Peter Kasting, 2014/07/28 19:20:50]

Removing the global one worked because I manually fixed all the errors that it
triggered before I landed the change to remove it.  Ideally individual targets
would not override this, but even if they do, this means we still have the
warning enabled for the majority of our codebase, which is progress over where
we were before.
Disabling it globally in GYP would be strictly worse than today, since that
would mean we won't get the warning coverage for all the targets that don't
individually disable it.

Can you change this to match the GYP build, i.e. re-enable this globally and
then disable it per-target instead?  Or if there's a list of the relevant
targets, send that to me to update, or something?

Basically, let's work together to make sure this is on by default instead of
off.

+      # 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