media/gpu: use new md5 API in VideoFrameValidator

We're getting rid of base/hash/md5, and migrating existing clients that
need to keep using md5 for now to crypto/obsolete/md5. This change
migrates MD5VideoFrameValidator to the new API. In the process, this
resolves all the remaining unsafe buffer warnings in
video_frame_validator.cc and enables unsafe buffer checking for that
file.

Bug: 428022614,406729261
Change-Id: Ib5599534ed650ba2554117f712fdabd67b538b47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6681210
Reviewed-by: Andres Calderon Jaramillo <andrescj@chromium.org>
Commit-Queue: Elly FJ <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1480344}