SDK / exoplayer

In CSS, ids are references using #. The absence of # references elements.

NOTE: If the id of a cue was "1", we support its reference with ::cue(#1).
In CSS, however, this is not valid, and the number should be escaped with
\3 as in ::cue(\31). We still do not use number escaping (and I doubt
whether we should at some point).
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=119634708