Use angle::FlatUnorderedSet for ImageSibling::mSourcesOf As my test with Chromium on Android, ImageSibling::mSourcesOf only contains one item, so change it to angle::FlatUnorderedSet with default size 2. Bug: chromium:1336126 Change-Id: Ie1ba3ebb195170ad5c9f4713b599a91de5052702 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3828444 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Auto-Submit: Peng Huang <penghuang@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>