This enables other implementations, such as streaming to memory
instead of a file.

BUG=swiftshader:9

Change-Id: I2a780ee67e9bccd157c120b7a0895d9764117464
Reviewed-on: https://chromium-review.googlesource.com/384911Reviewed-by: Jim Stichnoth <stichnot@chromium.org>
Tested-by: Nicolas Capens <nicolascapens@google.com>