Revert "[standalone] Remove the fallback root certificates."

This reverts commit 039aef728a.

Reason for revert: b/515601247

Original change's description:
> [standalone] Remove the fallback root certificates.
>
> The discovery of the system root certificates is more robust after 2142d8e148.
>
> TEST=ci
> Change-Id: I8215cb92f98e620b82f21261c13e061a2b488b5d
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/433923
> Reviewed-by: Slava Egorov <vegorov@google.com>
> Commit-Queue: Ryan Macnak <rmacnak@google.com>

TEST=ci
Change-Id: Ib07de221ff1160461d930f7165a7cd8179b4e3f3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/506100
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

tools/gn.py

@@ -233,6 +233,11 @@ def ToGnArgs(args, mode, arch, target_os, sanitizer, verify_sdk_hash,
     else:
         gn_args['dart_snapshot_kind'] = 'app-jit'
 
+    # 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
+
     if gn_args['target_os'] == 'linux':
         if gn_args['target_cpu'] == 'arm':
             # Default to -mfloat-abi=hard and -mfpu=neon for arm on Linux as we're