Use OS-provided trusted root certs on Linux

runtime/bin/root_certificates_unsupported.cc

Index: runtime/bin/root_certificates_unsupported.cc
diff --git a/runtime/bin/root_certificates_unsupported.cc b/runtime/bin/root_certificates_unsupported.cc
index a899e3c2a5b39364f87431ae3b96b95290ae2737..304a59e4bb0b13972bc144d4ed50494b4cfc9a72 100644
--- a/runtime/bin/root_certificates_unsupported.cc
+++ b/runtime/bin/root_certificates_unsupported.cc
@@ -5,7 +5,9 @@
 #if !defined(DART_IO_DISABLED) && !defined(DART_IO_SECURE_SOCKET_DISABLED)
 
 #include "platform/globals.h"
-#if defined(TARGET_OS_MACOS) || defined(TARGET_OS_ANDROID)
+#if defined(TARGET_OS_MACOS) || \
+    defined(TARGET_OS_ANDROID) || \
+    defined(DART_IO_ROOT_CERTS_DISABLED)
 
 namespace dart {
 namespace bin {
@@ -16,7 +18,7 @@ unsigned int root_certificates_pem_length = 0;
 }  // namespace bin
 }  // namespace dart
 
-#endif  // defined(TARGET_OS_MACOS) || defined(TARGET_OS_ANDROID)
+#endif  // defined(TARGET_OS_MACOS) || defined(TARGET_OS_ANDROID) || ...
 
 #endif  // !defined(DART_IO_DISABLED) &&
         // !defined(DART_IO_SECURE_SOCKET_DISABLED)