Memory-backed block cache.
At initialization, allocates memory needed to store the expected content
length.
If MediaCache attempts to write/move beyond the expected size, we grow the
buffer accordingly, as we cannot fully trust HTTP headers. (Future patch will
ensure we put a limit to this growth.)

MozReview-Commit-ID: GHxYMGXYrwI

--HG--
rename : dom/media/MediaBlockCacheBase.h => dom/media/MemoryBlockCache.h
extra : rebase_source : 4fe263006839ba82a77d124f147adf5943cfa651