The conversion from NV12 and other Bi or Tri planar formats, differs only in the UV handling.  The helper function supports passing a NULL for the dst_y channel indicating you only want to do the UV conversion.

TBR=harryjin@google.com
TEST=LibYUVConvertTest.NV12ToI420_NullY (601 ms)
BUG=libyuv:626

Review URL: https://codereview.chromium.org/2276703002 .