Fix incorrect assertion in SwapChain11.cpp. BUG=angleproject:1144 Change-Id: I6186381d1aad13bbb07af47b3cf94d6ec7cbb566 Reviewed-on: https://chromium-review.googlesource.com/405693 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>