The code currently pads the decoded frame width and height to a
multiple of 8 luma pixels, but there is a TODO suggesting that
this may be changed to only require a multiple of 4 in future.

But, as per the comments on the linked bug, there are good reasons
to keep the decoded width and height as multiples of 8. So delete
the outdated TODO and instead outline the reasons why the current
behaviour is helpful.

BUG=aomedia:727

Change-Id: I2340bbcd740afe74c2e6fb3cf2e7a420db2b4f40