Add partitionAllocGetSize() for determining actual size of allocation

Since PartitionAlloc allocates from buckets of quantized sizes, it will
sometimes allocate a significantly larger block than requested.  Add the
utility function partitionAllocGetSize() for retrieving the actual size
of an allocation, useful if the client can make use of the extra size.

BUG=

Review URL: https://codereview.chromium.org/169523004

git-svn-id: svn://svn.chromium.org/blink/trunk@167582 bbb929c8-8fbe-4397-9dbb-9b2b20218538