class Writer

This interface is used to write all packets generated by this consumer. Packets are written in

increasing order by start frame. Underflows will lead to gaps between packets. When the

consumer is stopped, it calls `End()` after writing the final packet. If the consumer is

started again, it starts writing more packets (there is no explicit "start" signal). This

interface is implemented by various kinds of audio sinks, such as packet queues and ring

buffers.

Consumers always write data on integral boundaries. Hence these functions use `int64_t` frames

instead of `Fixed` frames to avoid ambiguity and unnecessary `Floor()` operations.

TODO(https://fxbug.dev/42066188): Further clarify the semantics: is start_frame allowed to go

backwards?