Code review comment for lp:~kdub/mir/require-streams-when-creating-surface

The name added_streams is a bit confusing since streams may later be removed as well. How about just calling it 'streams'.
180 + std::unordered_map<SurfaceId, frontend::BufferStreamId> added_streams;
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We should just call them add_buffer_stream_for() and remove_buffer_stream_for() instead of
170 + void add_content_for(frontend::SurfaceId id, frontend::BufferStreamId);
171 + void remove_content_for(frontend::SurfaceId, frontend::Session&);