Check in //third_party/root_certificates.

No one could remember why this code was in a separate repo, and
checking it in to the SDK repo will make eliminating the
flutter buildroot repo a tiny bit easier.

TEST=It builds.
Change-Id: Ia34ca0c284fee1dd1734f45ef7acb9dd5cde808c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/342083
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>
Reviewed-by: Brian Quinlan <bquinlan@google.com>

runtime/bin/BUILD.gn

@@ -488,7 +488,7 @@ template("dart_io") {
 
     if (is_linux || is_win || is_fuchsia) {
       if (dart_use_fallback_root_certificates) {
-        sources += [ "//third_party/root_certificates/root_certificates.cc" ]
+        deps += [ "../../third_party/fallback_root_certificates" ]
       } else {
         defines += [ "DART_IO_ROOT_CERTS_DISABLED" ]
       }