Today MojoCdmAllocator keeps at most 3 free buffers. We should understand what's the implication of this for performance and memory usage. Here are some ideas:
- Add UMA about number of times that we need to allocate new buffers, and/or how 
- Hook up with MemoryPressureMonitor such that if memory pressure is high, drop those free buffers.
- Hook up with base::trace_event::MemoryDumpProvider so we can add perf test to monitor our memory usage.