Enable EXT_discard_framebuffer in ANGLE's D3D11 backend. This doesn't seem to cause the video-related WebGL test failures anymore that were previously seen. BUG=497445 Change-Id: I705ae8735823ab7f4b26cf7696cdb746936e4447 Reviewed-on: https://chromium-review.googlesource.com/308001 Tryjob-Request: Kenneth Russell <kbr@chromium.org> Tested-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>