Call vaInitialize() at PreSandbox stage.

This commit is not upstreamable, check details at
https://codereview.chromium.org/15955009/.

Remove hard code of va driver, instead call vaInitialize() before
sandboxing so that va driver can be loadded on different Linux distros.

BUG=244386
TEST=content_shell and chrome to view H264 oneline videos.

It can be mreged with previous libva patchset
d660a3e Enabling Hardware Acceleration with Libva in linux-x86.