Vaapi decode: extract ShouldDecodeOnclientPictureBuffers()

Cleanup: extract ShouldDecodeOnclientPictureBuffers(), which
is used in e.g. relanding crrev.com/c/1356019, crrev.com/c/1348869.
No new code intended, just extracting that method, that was
already reviewed and landed.

TBR=hiroh@chromium.org

Bug: 909926, 910986, 912295
Change-Id: I24112e737b882484802e5236235164490360ee6f
Reviewed-on: https://chromium-review.googlesource.com/c/1363796
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614089}
1 file changed