Simplify DefaultAllocator::Quantizer using partitionAllocActualSize()

Drop QuantizedAllocation completely, and replace with a much simpler
implementation that lets PartitionAlloc just round the allocation size
upwards to fit the nearest bucket.

R=cevans@chromium.org
BUG=

Review URL: https://codereview.chromium.org/177213007

git-svn-id: svn://svn.chromium.org/blink/trunk@167870 bbb929c8-8fbe-4397-9dbb-9b2b20218538