Due to legacy reasons, libyuv uses a root_dir variable in the DEPS file.
It was used to make the bots checkout into a 'src' dir instead of 'libyuv',
in order to get some tools working that had hard-coded assumptions on the name
'src' of the top-level directory. This changes all those dirs to be named
'src', which is needed to get tools_libyuv/autoroller/roll_deps.py to work
while keeping the buildbot infrastructure working.

BUG=libyuv:690
TBR=fbarchard@chromium.org

Change-Id: Ia138264d2576e3a9db95efa3de72a14531c92752
Reviewed-on: https://chromium-review.googlesource.com/447913Reviewed-by: Frank Barchard <fbarchard@google.com>