Use angle::FlatUnorderedSet for ImageState::targets In mose cases, chrome only needs 1 or 2 targets for an EGLImage, so using angle::FlatUnorderedSet with 2 default size instead of the std::set for targets. Bug: chromium:1336126 Change-Id: Ieff3cfbd5d020fda3aaecb162ddb354def14d962 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3827681 Auto-Submit: Peng Huang <penghuang@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>