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

This matches what the GYP build does.

BUG=398202
TEST=none

Review URL: https://codereview.chromium.org/1246103007

Cr-Original-Commit-Position: refs/heads/master@{#340590}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d408e1965f8e03629738039289ab7779c42a318c

config/compiler/BUILD.gn

@@ -723,11 +723,6 @@ 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