[Cronet] Don't spam log if system cookie doesn't have creation time.

ios/net/BUILD.gn

Index: ios/net/BUILD.gn
diff --git a/ios/net/BUILD.gn b/ios/net/BUILD.gn
index 8a77282cf2bbdc0b6b26e4d129f68eee4e5611f1..71322769db82ffba3e1dac93a03bc282ad643193 100644
--- a/ios/net/BUILD.gn
+++ b/ios/net/BUILD.gn
@@ -2,10 +2,17 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/buildflag_header.gni")
 import("//ios/build/config.gni")
+import("//ios/features.gni")
 import("//testing/test.gni")
 import("//url/features.gni")
 
+buildflag_header("ios_net_features") {
+  header = "ios_net_features.h"
+  flags = [ "CRONET_BUILD=$is_cronet_build" ]
+}
+
 group("all_tests") {
   testonly = true
   deps = [
@@ -15,6 +22,7 @@ group("all_tests") {
 
 source_set("net") {
   deps = [
+    ":ios_net_features",
     "//base",
     "//net",
     "//url:url_features",