The GMP cache is global right now, but the Shmems are only shared with a specific GMPParent/plugin. Move the data storage to the GMPParent while leaving the impl in the GMPVideoEncoder/etc - The Alloc/FreeShmem's are only defined at that level. Also adjust a few constants, and cleanup.

Note: applies on top of bug 1043531's patch

Comment on attachment 8463082 [details] [diff] [review] Move GMP shared mem cache from a global to per-GMPParent Approval Request Comment [Feature/regressing bug #]: 1020090 [User impact if declined]: Hopefully none. [Describe test coverage new/current, TBPL]: GMP plugin tests are in m-c and ready to land on Aurora. [Risks and why]: If declined, less similarity between Aurora and Nightly, where most of the testing and fixes are generated. Similar code reduces the risk of diverging bugs during this feature-polishing/release phase, where heavy QA is going on. It also eases uplifting any bugfixes. It *should* be ok without this patch, as with a single plugin this should work the same as without the patch - but we haven't tested without it. [String/UUID change made/needed]: none