Adds a method that forces the CPU flags. Useful when using libyuv inside
a sandboxed process which may not have access to the file system.

Bug: libyuv:787
Change-Id: I01f71e39a7301085d9de388eba930b4cac0fd7be
Reviewed-on: https://chromium-review.googlesource.com/972338
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: Frank Barchard <fbarchard@chromium.org>