I'm having a crack at this now and I've got it working using a vdpau-wrapper of
sorts, though there has been some scrambling of the colours. I'm still fixing
it.

What I have seen is that Chrome expects a feature that's not implemented in the
gallium state tracker:

https://groups.google.com/a/chromium.org/forum/#!topic/chromium-os-dev/HYz_MOfNtkE

"2. While searching for a unit test. I bumped into a bunch of test cases from
Autotest suite. I tried to run video_VideoDecodeAccelerator.h264
But the test case failed since one of VA-APIs used by chromium has not been
implemented in state_trackers yet (vaQuerySurfaceAttributes)
Is there any other unit test available, that I can run to confirm whether
decoding is happening properly? Maybe something that decodes and dumps raw data
into a file?
"

Chrome will not work directly with the mesa vaapi interface until this feature
is added.

I've found a patch for this feature into libva-vdpau-driver that works around
it not being there. It's included on the arch linux link above.

I could attach a tar the ebuild for gentoo but this user uses fedora.

Gentoo and arch Linux have similar patches. Ubuntu and fedora have their own
overlay's for the chromium vaapi wrapper.

Here's a fedora user's chromium build:
https://github.com/biswasab/chromium_vaapi_fed

I'm recompiling Chromium with swiftshader disabled to see if I can get around
the issues I'm having.

https://bugs.chromium.org/p/chromium/issues/detail?id=719213