The vpx_codec_decode() and aom_codec_decode() functions have a
void *user_priv input parameter. This input argument is copied to
the user_priv field of the vpx_image_t or aom_image_t returned by
vpx_codec_get_frame() or aom_codec_get_frame() function.

Chromium passes either the address of a timestamp local variable or
the a timestamp cast to a void* pointer as the user_priv argument to
vpx_codec_decode() and aom_codec_decode(). When I inspected how the
user_priv value is used, I found that it does nothing but to check
that libvpx or libaom copies that value to vpx_image_t or aom_image_t
correctly. Therefore, Chromium can just pass nullptr as the user_priv
argument, like all other users of libvpx and libaom that I've
inspected.

If Chromium cannot pass nullptr as the user_priv argument, that will
imply vpxdec.c and aomdec.c, the sample decoder program of libvpx and
libaom, have a bug, because both of them pass nullptr as the user_priv
argument. That will be very bad.

I will upload a CL shortly, which will show the relevant code in
chromium/src/media/filters/.

Note: Another potential issue I found is that Chromium does not call
vpx_codec_decode() and aom_codec_decode() with data=nullptr and
data_sz=0 to flush the decoder. I don't know if this is a problem.