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AtomicString statics are bound to the thread where they were
initialized. This change switches MSE implementation to instead use
enums or non-thread-specific strings, in anticipation of upcoming usage
of MSE in either the main thread or dedicated worker threads. Longer
term, similar will be needed for core/html/track objects, though we may
instead switch to using prepopulated, valid-everywhere, AtomicStrings at
that point (when the cross-thread MSE implementation has stabilized enough to
get AudioVideoTracks feature working fully when using MSE in worker).

BUG=878133

Change-Id: I37f973d5175deb7bce6b96106e0a30bd6fa15ceb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417452Reviewed-by: Will Cassella <cassew@google.com>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808549}