We should be using root_out_dir, not root_build_dir. This was causing
issues with NaCL. GN descriptions:

root_out_dir: [string] Root directory for toolchain output files.
root_build_dir: [string] Directory where build commands are run.

Bug: dawn:283
Change-Id: I0730b96d303151b715672d6937031e557e1994f0
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38748Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Jamie Madill <jmadill@chromium.org>