Update README.chromium to reflect recent NDK/SDK changes. BUG=599327 Change-Id: I6b6cc8da462dba0c52bb91828938796231a8ffaa Reviewed-on: https://chromium-review.googlesource.com/392806 Reviewed-by: John Budorick <jbudorick@chromium.org> Tested-by: John Budorick <jbudorick@chromium.org>