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This removes the VideoTrackWriter interface so that it will not become
visible when the WebCodecs feature is enabled.

The VideoTrackReader unittest depends on the VideoTrackWriter C++ code,
so that has been left for now.

Bug: 897297
Change-Id: I1c328d7ca995cf9019ca50c65f8cb7257fbb06ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2353538
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Auto-Submit: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797945}