Quanitize allocations in partitionAllocGeneric.

This prevents fragmentation (and therefore memory waster) of various
kinds.

Also use 8-byte granularity for allocations <128 bytes, since allocation
sizes 24, 40 are _really_ common for things like StringImpl.

BUG=270531

Review URL: https://codereview.chromium.org/26067002

git-svn-id: svn://svn.chromium.org/blink/trunk@158978 bbb929c8-8fbe-4397-9dbb-9b2b20218538