Restore root_dir variable default.
Restore root_dir variable default.

Its default value was accidentally cleared in
https://chromium-review.googlesource.com/436264
This restores it. It's set on the bots, which is
why it wasn't noticed.

BUG=libyuv:676
TBR=fbarchard@google.com

Change-Id: Ifad560e9e5caa134fef4c2cbaf15f1f3bfa6028d
Reviewed-on: https://chromium-review.googlesource.com/438084
Reviewed-by: Henrik Kjellander <60c11d3e646624f25a02b88f51d51ca6b7ff7c72@chromium.org>

lemenkov/libyuv,lemenkov/libyuv,lemenkov/libyuv,lemenkov/libyuv