Introduce vaLockBuffer APIs in libva.

Reason that add this change:
1080p video playback on Tizen is quite unsmooth on VTC-1010, with only 15 render
fps and ~70% CPU usage. Ozone-wl landed a workaround solution that enables a
zero buffer copy method that uses libva vaLockBuffer APIs which gains much better
performance than previous method.
https://github.com/01org/ozone-wayland/pull/260

In order to make the change also benifit Crosswalk/Tizen, we should add the
vaLockBuffer API declarations inside Chromium Crosswalk to compile
the code successfully.

The long term goal of our work would be landing those API declarations inside
Chromium upstream, after the API implementations are landed in libva and
libva-driver upstream.

Therefore, this CL is a temporary solution. It would be reverted after the long
term goal is achived.
Merge pull request #183 from shaochangbin/vaLockBuffer

Introduce vaLockBuffer & vaUnlockBuffer APIs in libva.