Adds an option to split chroma tx size by one level if the luma
transform size is split at least one level. Otherwise use the
largest transform size for chroma.
This option is enabled with DISABLE_VARTX_FOR_CHROMA set as 2,
but is currently turned off until we finish testing.

Change-Id: Ie048c4e33193e0cc7e1bbee37e4ccc085834433c