Require 8 stencil bits in the BlitFramebufferANGLETest. The test can fail if it uses a config where the default framebuffer does not have a stencil buffer. BUG=angle:658 Change-Id: I82eaf5b508182514439e69469e77591e1aceaf66 Reviewed-on: https://chromium-review.googlesource.com/246461 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>