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/438084Reviewed-by: Henrik Kjellander <kjellander@chromium.org>

DEPS

 vars = {
   # Override root_dir in your .gclient's custom_vars to specify a custom root
   # folder name.
-  'root_dir': Var('root_dir') + '',
+  'root_dir': 'libyuv',
   'chromium_git': 'https://chromium.googlesource.com',
   'chromium_revision': '8346af5a710b5c692803beab335b5e5439883387',
   'swarming_revision': 'ebc8dab6f8b8d79ec221c94de39a921145abd404',