Switch H264SEIMessage and H265SEIMessage to use absl::variant

1. C++ unions are quite fiddly and hard to hold correctly. A variant is
   much more typesafe.
2. A followup CL removes the use of `memset()` to initialize C++ structs
   in favor of using in-class initializers. Without migrating to
   variants, it's a bit trickier to ensure that structs containing
   unions are properly initialized. Getting this wrong means subtle and
   hard-to-debug test failures.

Change-Id: I1d36954151afde2c3e45f647091345ae2f7c0927
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6307175
Reviewed-by: Eugene Zemtsov <eugene@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1426538}