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Make sure we use the same file in all instances of VaapiWrapper.

Define a refcounted global instance of VADisplayState that is only initialized
(via vaInitialize()) once and destroyed upon release of the last refcount.
This ensures a single drm_intel_bufmgr per fd, and avoids the double-close
problem as seen in bug 464628.

Pipe a PreSandboxInitialization to open the render node before starting sandbox.

After this CL the VEA/VDA unittests need to invoke the PreSandboxInitialization
as well.

BUG=475250
TEST=verify HW video works on link (check histograms) and VDA/VEA unittests

Review URL: https://codereview.chromium.org/1137483002

Cr-Commit-Position: refs/heads/master@{#329410}