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This CL contains some minor cleanup in the video decoder tests:
- Update an outdated comment.
- Remove a duplicate define that's already defined in macros.h.
- Improved debug output when waiting for events.
- Moved some initialization to header file.

TEST=./video_decode_accelerator_tests on nocturne

BUG=None

Change-Id: Id0794cff6036396e46e293077663b62bba0b3560
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032464Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: David Staessens <dstaessens@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742064}