This requires you don't have target_os=["ios"] set in your
libyuv root .gclient file, since that will make native_client not
being downloaded due to
https://code.google.com/p/chromium/codesearch#chromium/src/DEPS&l=357

BUG=libyuv:573
TESTED=
rm chromium/.last_sync_chromium
rm chromium/.gclient.tmp_entries
gclient sync
native_client/build/gyp_nacl all.gyp -Dgtest_target_type=executable -Dmsan=0 -Duse_system_yasm=0
ninja -C out/Debug

Review URL: https://codereview.chromium.org/1845003004 .