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

third_party/protobuf/BUILD.gn

Index: third_party/protobuf/BUILD.gn
diff --git a/third_party/protobuf/BUILD.gn b/third_party/protobuf/BUILD.gn
index 7d42a00dd6a9c36da573ed5c9b31528ec60410aa..0c57bd65c46710a94e5821bc09554f2893664092 100644
--- a/third_party/protobuf/BUILD.gn
+++ b/third_party/protobuf/BUILD.gn
@@ -77,11 +77,11 @@
 protobuf_lite_cflags = []
 if (is_win) {
   protobuf_lite_cflags = [
-    "/wd4018",  # Signed/unsigned mismatch in comparison.
-    "/wd4244",  # Implicit conversion, possible loss of data.
-    "/wd4355",  # 'this' used in base member initializer list.
-    "/wd4267",  # Size_t to int truncation.
-    "/wd4291",  # No matching operator delete for a placement new.
+    "/wd4018",  # signed/unsigned mismatch in comparison
+    "/wd4244",  # implicit conversion, possible loss of data
+    "/wd4355",  # 'this' used in base member initializer list
+    "/wd4267",  # size_t to int truncation
+    "/wd4291",  # no matching operator delete for a placement new
   ]
 }