Document thread-unsafe iterator access to resource maps By using a proxy type, everywhere resource maps are iterated are clearly marked as not being thread safe. In most cases, only destruction and capture/replay iterate over these maps, which means thread safety is not an issue (or is externally enforced). The only case where iterators are used in the presence of other contexts is with ANGLE_request_extension, which is changed to explicitly require the application to ensure thread safety. In practice, the user is Chrome which already guarantees this. Bug: angleproject:8667 Change-Id: I7af13c6433b6955d9c36f9088b3aa4c065e1cfc1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5526428 Reviewed-by: Charlie Lao <cclao@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>