Bug 1371882 - MemoryBlockCache is responsible for tracking the combined size of all its buffers - r=cpearce

MemoryBlockCache won't allow initializing, or growing an existing buffer,
above the limit (min of 'media.memory_caches_combined_limit_kb' or
sysmem*'media.memory_caches_combined_limit_pc_sysmem'/100).

MozReview-Commit-ID: 6MkwFp2eeth

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extra : rebase_source : 17345f6fe9f00fddfbef87090665afccaabb2cf5