Add partitionAllocActualSize() for predicting actual size of allocation

Similar to partitionAllocGetSize() but can be used before the allocation
has been performed.  Falls back to returning the size unchanged for
"impossible" cases, like when PartitionAlloc isn't actually used, or when
the size is too large to allocate.

R=cevans@chromium.org
BUG=

Review URL: https://codereview.chromium.org/172533004

git-svn-id: svn://svn.chromium.org/blink/trunk@167644 bbb929c8-8fbe-4397-9dbb-9b2b20218538