Since MemoryBlockCache is used only when the content length is smaller than
MediaPrefs::MediaMemoryCacheMaxSize() * 1024, we should set mMaxBlocks according
to the pref value to prevent blocks from being evicted unexpectedly.

MozReview-Commit-ID: BaI0A0VUbkv

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extra : rebase_source : 252d469c101cafe3fed51a759c3fac63db5f161a
extra : source : 8aa941397a3d2a354689c1e9d0a5a18bb659366a