GN: Reset default for fallback root certs to false.

tools/gn.py

Index: tools/gn.py
diff --git a/tools/gn.py b/tools/gn.py
index 91caa191389008aa18e1fc100ed2930030803e9c..fd9a1b74cd00343fde872e79b1ad042c397ad6a5 100755
--- a/tools/gn.py
+++ b/tools/gn.py
@@ -77,6 +77,11 @@ def to_gn_args(args, mode, arch, target_os):
   # dart_bootstrap if the prebuilt SDK doesn't work.
   gn_args['dart_host_pub_exe'] = ""
 
+  # We only want the fallback root certs in the standalone VM on
+  # Linux and Windows.
+  if gn_args['target_os'] in ['linux', 'win']:
+    gn_args['dart_use_fallback_root_certificates'] = True
+
   gn_args['dart_zlib_path'] = "//runtime/bin/zlib"
 
   # Use tcmalloc only when targeting Linux and when not using ASAN.