Move Initialize() to VaapiImageDecoder parent class.

This CL moves the implementation of Initialize() to VaapiImageDecoder,
since it is common to all implementing classes.

Bug: 877694
Test: jpeg_decode_accelerator_unittest
Change-Id: Ic99601953ae1c7a572ba8a0b0bf43675b2b0969d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1654249
Commit-Queue: Gil Dekel <gildekel@chromium.org>
Reviewed-by: Andres Calderon Jaramillo <andrescj@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668645}