Make "force context check" an optional GN flag. Previously it was enabled by default in every build that includes D3D11. This would impact CPU overhead in Chrome and other targets that don't need this feature. Bug: angleproject:5828 Change-Id: Ic9d700847c72978540b66fdaf7a62f959779f2e6 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2809855 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Doug Horn <doughorn@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>