Without this, `gn gen --check` for Chromium would fail with errors like:

  $ gn gen --check out/gn
  ERROR at //cc/resources/video_resource_updater.cc:25:11: Can't include
  this header from here.
  #include "third_party/libyuv/include/libyuv.h"
            ^----------------------------------
  The target:
    //cc:cc
  is including a file from the target:
    //third_party/libyuv:libyuv_internal

  [...]

  Dependency chain (there may also be others):
    //cc:cc -->
    //third_party/libyuv:libyuv --[private]-->
    //third_party/libyuv:libyuv_internal

BUG=libyuv:685, chromium:692600

Change-Id: I5e5bde2f0e4802d70bfd438793eb2460437398ee
Reviewed-on: https://chromium-review.googlesource.com/449934Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Henrik Kjellander <kjellander@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>